Steel Quenching Technology.pdf

Steel Quenching Technology

Introduction “Quenching is one of the least understood of the various heat treating

technologies,” in the words of a world authority on the subject, George E. Totten of Union Carbide Chemical & Plastics Co., Inc.

Other dimensions of the problem are identified in the follouing quotes from other experts:

l “Quenching is critically important, but often the neglected part of heat treating.”

l “Quenching is the most critical part of the hardening process... [it] must be designed to extract heat from the hot horkpiece at a rate required to produce the desired microstructure, hardness, and residual stresses.”

l “Distortion is perhaps one of the biggest problems in heat treating... little information on the subject has been published.”

The subject was introduced in the previous chapter by two articles on the subject: “Causes of Distortion and Cracking during Quenching” and “Stress Relief Heat Treating of Steel.”

In this section, the topic is surveyed in depth in eight articles on conventional quenching processes:

l Air quenching l Water quenching l Oil quenching l Polymer quenching l Molten salt quenching l Brine quenching l Caustic quenching l Gas quenching

In addition. I7 alternative methods of quenching are discussed in articles:

l Austempering l hlartempering

l Isothermal quenching l Aus-bay quenching

l Spray quenching l Fop quenching l Cold die quenching

l Press quenching

l Vacuum quenching

l fluidized bed quenching

l HIP quenching

l CUtrasonic quenching l Quenching in electric and magnetic fields l Quenching flame and induction hardened parts

l Self-quenching processing-electron beam hardening, laser hardening, and high frequency pulse hardening

References

I. George E. Town. preface to AShl Conference Proceedings, “Quench- ing and Distortion Control.” ASM International, ‘92

2. Totten et al, “Handbook of Quenchants and Quenching Technology,” ASM International. ‘93

Air Quenching Process I. Air is the oldest most common. least expensive quenching medium, Ref

Characteristics of Process Air, a gas high in nitrogen, cools by extended vapor phase cooling.

Operating Information As with other quenchants. heat transfer rates are dependent on flow

rate-in this instance, flow rate of air past the hot part (see Figure). Cooling can be speeded up by increasing the velocity of air 110~. but the accelerated rate is not sufficient to quench harden man) steels. The ability of air to harden plain carbon steels drops dramatically with increasing carbon content (see Figure).

Application Range Air is used in quenching steel and several nonferrous metals. The

comparative heat transfer coefficients of different metals as a function of surface temperature are shoun in an adjoining Figure. To get optimal hardness, it is often necessary to use a more actike quenching medium. such as brine or oil.

Reference I. Totten et al, Handbook oj Qwttclrottts md Qutwclriny Teclrtrolog~:

ASM International. 1993

Heat transfer coefficients for air cooling as a function of surface temperature

78 / Heat Treater’s Guide

Cooling capacities of still and compressed air Hardness values obtained with different quenching media

Water Quenching Process Like air, water is an old, common, inexpensive quenchant (Ref I). It is

applied in several ways: in straight, immersion quenching; in a special, double-step, hot water quenching process; and in conjunction with polymer quenchants and with brine quenchants.

Characteristics of Process Water, especially cold water, is one of the most severe quenching media

available. Vigorously agitated water produces a cooling rate approaching the maximum with liquid quenchants (Ref 2). As water temperature rises, the vapor phase is prolonged and the maximum rate of cooling drops sharply (see Figure).

Operating Information Generally, good results are obtained in straight immersion quenching by

maintaining water temperatures in the range of I5 to 25 “C (60 to 75 “F) and by agitating water to velocities greater than 0.25 m/s (50 ft/min). Water temperature, agitation of water, and amount of contamination in the water must be controlled. The comparative cooling properties of hard water and distilled water are indicated in an adjoining Figure.

The detrimental effect of temperature dependence and vapor phase stability can be minimized (Ref 3) by:

l Maintaining water at a low temperature through cooling l Vigorous agitation to disperse the vapor blanket l Addition of an organic salt (see Brine Quenching)

Double-step, hot water quenching is a possible alternative to lead patenting of steel wire. It consists of:

l Heating wire to 920 to 950 “C ( 1690 to 1710 “F) and immersing into boiling hot water for an appropriate time

l Removing wire from water and air cooling (Ref I)

Application Range Water is the choice where a severe quench does not result in excessive

distortion and cracking. Use generally is restricted to quenching simple, symmetrical parts made of shallow hardening grades of steel. Other applications include austenitic stainless steels and other metals that have been solution treated at elevated temperatures.

Effect of temperature on quenching properties of water. Source: ES. Houghton & Co.

Steel Quenching Technology / 79

Cooling curves for hard water and distilled water

References 2. ASM Metals Handbook, Heat Treating, Vol 4, 10th ed.. ASM lntema-

I. Totten et al., Handbook of Quenchanrs and Quenching Technology, tional. 1991

ASM International, 1993 3. Houghron on Quenching, E.F. Houghton & Co., Valley Forge, PA

Oil Quenching Process AU modem quenching oils are based on mineral oil, usually paraffin

based, and do not contain fatty oils. Usage of oils opens up a number of options for the heat treater:

l Normal-speed oil for treating steels high in hardenability l Medium-speed oils for medium hardenability steels l High-speed oils for treating low hardenability steels and for other appli-

cations l Hot oil quenching (also called marquenching or martempering) provides

another option l Water-washable quenching oils: for removing oils on treated parts with

plain water

Characteristics of Process Oils are characterized in various ways, depending upon operating re-

quirements. Quenching speed and operating temperature are among these considerations.

The importance of quenching speed is that it influences hardness and depth of hardening. Cooling rate curves for normal-, medium-, and high- speed quenching oils are shown in an adjoining Figure (Ref I). Cooling curves for different quenchants are given in an adjoining Figure (Ref 7).

Almost all quenching oils produce lower quenching rates than water or brine solutions, but they remove heat from workpieces more uniformly than water normally does, meaning less likelihood of distortion and cracking (Ref 3).

Temperature of operation is important because it influences:

l Oil life l Quenching speed l Viscosity of oil l Distortion of workpieces

Effect of temperature on quenching speed for a hot quenching oil is shown in an adjoining Figure (Ref I).

Changes in viscosity can indicate oxidation and thermal degradation, or the presence of contaminants. Ln general, viscosity goes up as an oil degrades and can result in changes in quenching speed.

Flash point, another consideration, is the lowest temperature at which oil vapors ignite in the presence of an ignition source; it is important because

Cooling rate curves for quenching oils. Source: E.F. Houghton & co.


Cooling rates for different quenchants

Effect of temperature on quenching speed of hot oil. Source: E.F. Houghton&Co.

Characteristics of Quenching Oils

Bath FlZL5b

Qpical GM viscosity quench* Elot- at 40 OC meter wire

temperature point (loooF), (nickel ball) test, ‘Qpe ofoil “C “F OC OF sus time, s A

Con\rntionaI ~65 <IS0 170 3-m 105 16.0 30 Accelerated <I20 <xi0 180 355 94 IO 39 hlarquenching COO <-u)o 300 570 7ocl 30 30

Use Temperatures for Marquenching Oils

Viscosity at 40 T U~W, SUS

hlinimum flash point

T OF

Use temperature Protective

Open air atmosphere T OF oc OF

250-550 “0 130 9s-150 X0-300 95-175 200-350 700 1500 2.50 -180 120-175 250-350 120-205 250-400 2000-2800 290 550 I SO-205 3OsmO I SO-230 300-450

section that require very high rates of cooling to get maximum mechanical properties

it is related to the maximum safe operating temperature-usually -IO to SO “C (71 to 90 OF) below the open cup flash point for oil.

Hot oil quenching: for applications where it is desirable to keep distortion and cracking to a minimum. It is a two-step operation:

Operating Information Data on the use temperatures for conventional, accelerated, and mm-

quenching oils are given in an adjoining Table and use temperatures for marquenching oils are given in a second Table (Ref 2).

Applications based on oil speed are as follows:

l Operating temperature generally of the oil is 100 to 200 “C (210 lo 390 “F). l Operating temperature is held until the temperature throughout the

workpiece is uniform. l Workpieces are then air cooled to ambient temperature.

l Normal-speed oils: used where the hardenability of a steel is high enough to provide specified mechanical properties with slow cooling. Typical applica6ons are highly alloyed steels and tool steels

l Medium-speed oils: typical applica6ons are medium- Lo high-hardenability steels

References

I. Houghton on Quenching, E.F. Houghton & Co., Valley Forge, PA 2. Totten et al.. Handbook oj’ Quenchants and Quenching Technology

ASM International, 1993 l High-speed quenching oils: for low hardenability alloys, carburized and 3. ASAl Metals Handbook, Hem Trearing, Vol -I, 10th ed.. ASM lntema-

carbonitrided parts, and medium hardenability steel parts large in cross tional. 1991


Polymer Quenchants Around 20 different aqueous polymers, it’s reported, have been used in

quenching steels (Ref I). They include:

. Polyvinyl alcohol (PVA) l Polyalkylene glycol (PAG) l Sodium polyacrylate (ACR) l Polyvinyl pyrrolidone (PVP) l Polethyl oxazoline (PEO)

PAG is No. I in usage today

Characteristics Inverse solubility in water is a key characteristic of a number of polymer

quenchants, including PAG’s, because this phenomenon modifies the conventional, three-stage quenching mechanism, providing flexibility in cooling rate. These polymers are completely soluble in water at room temperature, but insoluble at elevated temperatures, ranging from 60 Lo 90 “C (l-10 to I95 “F).

When a hot part is fist immersed in a quenchant bath, the quenchant in the immediate vicinity of the hot metal surfaces becomes insoluble and deposits itself on the part in the form of a polymer-rich film.

The tilm acts as an insulator, which slows down cooling to a rate analogous to that of oil in the vapor phase. ln a number of applications. the quenching rates of aqueous polymers are intermediate between those of water and oil (see adjoining Table).

Ln stage 2 of cooling (boiling phase), the film eventually collapses and the quenchant comes into contact with the hot metal. resulting in nucleate boiling and high heat extraction rates.

In the final stage, cooling is by conduction andcomection into the liquid. When metal surface temperatures fall below the inversion temperature. i.e.. 75 s, the polymer redissolves and forms a homogeneous polymer-water mixture.

Operating Information Cooling rates can be tailored to requirements by changing the concentra-

tion of the solution, quenchant temperature. and degree of agilation of the bath.

Concentration influences tilm thickness; with increasing concenfra- tion, the maximum rate of cooling and the cooling rate in the comection phase drop (see Figure). Agitation of the quenchant has little effect during the film stage.

Wettability of workpiece surfaces is improved with S% solutions of PAG, which is beneficial to quench uniformity. At this concentration, problems with soft spotting associated with water quenching are avoided.

Concentrations in the IO to 20% range accelerate cooling rates to the level of fast quenching oils. These concentrations are suitable for quenching low hardenability steels requiring maximum mechanical properties.

Concentrations of 20 to 308 boost cooling rates suitable for a wide range of through hardening and case hardening steels.

Bath temperature has an influence on the quenching speed of solutions. The effects of three different temperatures on a 2% PAG concentration with vigorous agitation is sho& n in an adjoining Figure. The maximum cooling rate decreases with increasing temperature. PAG solutions must bs

Typical Quench Severities Achievable with Various Media

Quenchant

Oil Polymer Water Brine

Grossmann H factor

03-0.8 o.L?- I.2 0.9-1.0 2.0.5.0

Effect of PAG concentration on quenching characteristics

Effect of PAG-water temperature on quenching characteristics

1 Effect of agitation on quenching characteristics of PAG solu- 1


Sensitivity band for quenchant selection

cooled to prevent them from reaching the inversion temperature. A maximum operating temperature of approximately 55 “C (130 “F) normally is recommended.

Agitation has an important effect on all polymer quenchants, by ensuring a uniform temperature distribution within the bath; it also affects cooling rate (see Figure). As the severity of agitation increases, the duration of the vapor phase (tilm phase) is shortened and eventually disappears. During the convection phase, agitation has comparatively little effect.

PAG’s are used in immersion quenching of steel parts, in induction hardening, and in spray quenching. Applications of other polymers include forgings, open tank quenching of high hardenability steels, use in integral quenching furnaces. patenting of high-carbon steel wire and rod, and in quenching railroad rails.

Guidelines for Selecting Polymers Considerations generic to polymer quenchants include:

l Material composition

l Section size of workpiece

l Type of furnace l Quenching system design

l Method of quenching

l Distottion control

Material composition and section size play critical roles in quenchant selection (see Figure). Note that this figure provides guidelines for selecting polymer quenchants based on such considerations as hardenability of the steel, section size, quench severity factors, and polymer selection per applications. Alloy content influences hardenability, which determines the quenching speed needed to get a specified hardness and other properties.

Spray curtain in quenching chute

Section size and complexity affect quenching speed requirements. Heavy section parts are quenched faster than those with thin sections to get equivalent results.

Furnaces used in oil quenching may require modification for polymer quenching, and certain precautions are observed.

The design of integral quench furnaces, for example, may require modification to minimize the possible effects of water vapor in furnace atmospheres. Changes include ensuring a good inner door seal and the mainte- nance of positive gas pressure in the hot zone.

Spray curtains are needed in the quenching chutes of continuous furnaces to prevent contamination of the furnace atmosphere with water vapor (see Figure).

A precaution: polymer quenching of steel parts previously treated in salt baths generally is not recommended due to the effects of the carryover of high-temperature salt.

Aqueous polymer quenchants are recommended for parts treated in induction heating.

Quenching system design can have an influence on quenching characteristics. Examples include design features relating to agitation, method of circulating quenchants. and fluid temperature controls.

Method of quenching can directly affect cooling rates and the results obtained in quenching.

Direct Quenching. This technique is commonly used in quenching with aqueous polymers in many different types of furnaces.

Time or Interrupted Quenching. This technique is used to change the cooling rate during quenching, i.e.. quenching a large forging in water for a specified time, then transferring the workpiece to a polymer quenchant to reduce cooling in the convection phase. An alternative practice, used to reduce quench cracking and distortion, is to quench first in a polymer solution, followed by an air quench.

In spray quenching quenchant characteristics are influenced by the volume and pressure of the quenchant and by spray nozzle design.

Distortion control is needed in quenching thin or complex section workpieces. Use of a slower quenchant is one of the control techniques.

References

I. Totten et al., Handbook of Quenchotm and Quenching Technology, ASM International, 1993

2. Horcghron on Qrtenching. E.F. Houghton & Co., Valley Forge, PA


Molten Salt Quenching Process These salts usually are the medium of choice for high-temperature

quenching. They are either binary or ternary mixtures of potassium nitrate (KNO3). sodium nitrite (NaNO?). and sodium nitrate (NaNO3). Ref I.

Characteristics Minimum quenching temperatures depend on the melting point of the

salt mixture, which depends on the composition of the mixture (see Fig- ure); the ratio of the salt mixture may also affect the viscosity of the medium, which, in turn, affects cooling.

Operating Information Quenching temperatures of the bath range from 140 to 600 “C (285 to

II IO “F), but salt melting points as low as 80 “C (175 “F) can be obtained with additions of up to 10% water.

Control of bath temperature is critical (see Table). Salt baths are subject to potential explosive degradation at temperatures above 600 “C (I I IO “F). Care is also advised in making water additions, because they are accompanied by spattering of the molten salt. In one safety procedure, additions are made very slowly and bath temperatures are held below 175 “C (3-U “Fj. An automatic additive device and probe monitoring system (see Figure) is an alternative. Controlled additions of specific amounts of salt are made in a temperature range of I80 to 250 “C (355 to 180 “F).

Another consideration: salt can absorb water from a humid environment at room temperature when a quenching system is not in use. Before normal operations are resumed, heating the bath to 95 “C (205 “F) until all water is removed is a general recommendation.

Effect of Salt Temperature on Quench Severity(a)

Salt temperature Grossmann H factor, hr.-r T OF Center Surface

195 385 0.46 0.63 200 390 0.45 0.65 230 450 0.40 0.65 270 51s 0.45 0.64 295 560 0.4 I 0.57 350 660 0.43 058

(a) A KNO,-NtiO, salt with a melting point of I35 “C (275 OF) was used u ith no agitation.

Degussa system for adding water to molten salt

Like all quenchants, the heat extraction capabilities of molten salt depend on the agitation rate of the bath (see Figure). Agitation is applied in several ways. One approach is shown in an adjoining Figure.

In heat treating it is common for steel to be austenitized in a high- temperature salt bath composed of a binary blend of KNO3/NaNO3, or a ternary chloride blend, such as KCVLiCVNaCI. When austenitization is completed, the workpiece is quenched in a lower temperature, ternary blend of KNO~/NaN02/NaN03.

Freezing points of ternary alkali nitrate-nitrite mixtures given in percent

Effect of agitation on quench severity of molten salt


Application Range Dual impeller salt bath agitation system Molten salt quenching applications include:

l Martempering and austempering (see items on both subjects in this chapter)

l Quenching high-alloy steels l Quenching high-speed steel tools, to minimize scaling, distortion. and

cracking l Quenching steels such as spring wire to reduce the risk of cracking

during martensitic formation l Quenching to enhance the formation of high-temperature transformation

products, such as bainite and ferrite.

Reference

I. Totten et al., Handbook of Quet~chuttts curd Q~renchitrg khnolog~~ ASM International, 1993

Brine Quenching Process The term refers to aqueous solutions containing different percentages of

salts such as sodium chloride (NaCI) or calcium chloride (CaClj.

Characteristics Cooling rates are higher than those of water for the same degree of

agitation, or. alternately, less agitation is needed to get a given cooling rate. Higher cooling rates reduce the possibility of steam, the cause of soft spots in quenching, but higher cooling rates generally increase the likelihood of distortion and cracking. Use of baffling patterns on quench tanks and propeller agitation may be needed in quenching very lower hardenability steels (Ref 2).

In quenching, minute salt crystals are deposited on the surfaces of workpieces. Localized high temperatures cause crystals to fragment vio-

Relation of Brine Density to Brine Concentration (Ref 2)

salt, ?o

Specitk gravity Direct reading

hydrometer OBk(a) Salt concentration

sir. lb/sd

NaCl solutions -I I .026&J 3.8 11.1 0.313

6 I.0413 5.8 62.4 OS2 I

8 I .0559 7.7 81.5 0.705 9 I .0633 8.7 95.9 0.800 IO I .0707 9.6 107.1 0.89-l

I2 I .0857 II.5 130.3 I.087

NaOB solutions

I 1.0095 I .-I IO.1 0.08-E

2 I .0207 2.9 20.4 0. I704

3 I.0318 4.5 31.0 0.2583 4 I .04x3 6.0 -11.7 0.348 I

5 I .OS38 7.3 52.7 0.1397

(a) “Be. Baumb; specific gravity for liquids hea\ ier than Hater is 1154 I15 -n). where II is reading on Be scale in “Be

lently. creating turbulence that destroys the vapor phase, resulting in very high cooling rates.

Operating Information Brine concentration is expressed in several ways (see Table). Both

sodium chloride and sodium hydroxide. the latter a caustic solution, are covered in the Table.

Brine concentrations up to 33 percent progressively reduce the vapor phase. but such concentrations generally are considered impractical. A IO percent solution of NaCl is quite effective in hardening. The relationship of brine concentration to hardness is indicated in an adjoining Figure. It is necessary to monitor brine concentration to get reproducible results in quenching.

Cooling properties are not seriously alTected by small variations in the operating temperatures. Brines can be used at temperatures near that of

Relation of hardness to brine concentration when still- quenching, end quench specimens 90 “C (195 “F) brine solution. Number above curves indicate distance from quenched end in UnitS Of ‘/lfj in. (Ref 2).


Relation of hardness to distance from quenched end of specimens quenched in water and brine. Cooling power than that of water at 80 “C (175 “F) (Ref 2).

. of brine is greater

boiling water, but their maximum cooling power is at a temperature of approximately 20 “C (70 “F). Effects of temperature on cooling power are indicated in an adjoining Figure.

Sludge and scale should be removed from baths periodically. They can clog pumps and recirculating systems and reduce cooling rates. Excess water reduces solution strength and cooling power.

References

I. Totten et al., Handbook of Quenchnnrs and Quenching Technology, ASM International. 1993

2. ASM Merals Handbook, Hear Treating, Vol 4, 10th ed.. ASM fntema- tional. 1991

Caustic Quenching Process The most common alternative to sodium chloride quenching is aqueous

sodium hydroxide (a caustic) in concentrations ranging from 5 to IO

percent (Ref I).

Characteristics Cooling rates are similar to those of sodium chloride at high surface

temperatures. Slower cooling rates than those available with sodium chloride are obtained in the martensitic transformation temperatures for many

steels (450 “C. or 660 “F), which would be expected to reduce suscepti- bility to cracking.

Operating Information The effect of NaOH concentration on cooling rate, at a bath temperature

of 20 “C (70 “F), is shown in an adjoining Figure. The effects of I to 5

percent concentrations of NaOH are shoun in an adjoining Table. In practice, aqueous solutions are in the 5 to IO percent range.

Comparatively, NaCl solutions are considered to be safer. less costly. and easier to handle than NaOH solutions. The main shortcoming of the latter is that its high alkalinity is harmful to human skin (Ref 2).

Relation of Brine Density to Brine Concentration of NaCl and NaOH Solutions

Salt, 9

Specific gravilg Direct reading Salt concentration

hjdrumeter “Be(a) ks lb/gal

NaCl solutions

-I I .0268 3.8 11.1 0.343 6 I.Wl3 5.8 62.3 0.52 I 8 I .055Y 7.7 84.5 0.705 Y 1.0633 8.7 95.9 0.800 IO I .0707 9.6 107.1 0.893 I2 I .0857 I IS 130.3 I.087

NaOEl solutions

I I.OO9.r I .-I 10.1 0.0842 2 I .0207 2.9 20.4 0.170-t 3 I.0318 4,s 3 I .o 0.2583 -I I .@I28 6.0 41.7 0.318 I 5 I .0538 7.4 52.7 0.1397

(a) “BC. Baumt: specific grak ity for liquids hea\ ier than \\ater is l-IS/( I45 -n). where II is reading on BP scale in “Be


Effect of NaOH concentration on cooling rate

References

I. Totten et al.. Handbook of Quenchants and Quenching Technology, 2. ASM Metals Handbook. Heat Treating, Vol 4, 10th ed., ASM Interna- ASM International, 1993 tional. 1991

Gas Quenching Process Atmospheres containing some hydrogen or helium are commonly used.

Nitrogen is among the alternatives. Gases are also used in vacuum quenching.

Characteristics Cooling rates are faster than those in still air and slower than those

obtained with oil. Austenitized workpieces are placed directly in the quenching zone and heat is extracted by a fast-moving stream of gas (Ref I).

Operating Information The cooling rate of the metal being treated is related to surface area and

mass of a part, as well as the type, pressure, and velocity of the cooling gas.

Cooling curves in quenching 4130 steel in gas, oil, air (normalizing)

and still

Cooling rate is adjusted and controlled by altering the type, pressure, and velocity of the gas.

In quenching, large volumes of gas are directed through nozzles or vanes to impinge on the workpiece. After the gas absorbs heat from the hot workpiece, it is cooled by being passed through water-cooled or refiiger- ated coils. Recirculating fans return gas to the nozzles, through which they are again directed at the workload.

Quenching units are of the batch or continuous types, and the former are most commonly used.

Brinnel hardness of forged, 1095 steel disks 100 mm (4 in. thick) after oil quenching, gas quenching (forced air), and cooling in still air (normalizing)


Application Range In some instances quenching in still gas is too slow and oil quenching is

not desirable for such reasons as distortion, cost, handling problems, or insufficient ftnaf hardness. Quenching in a fast-moving stream of gas is a compromise.

The process is used, for example, in hardening aircraft tubing, steels that are not air hardenable, and tool steels.

Data on quenching 4 I30 aircraft tubing are found in an adjoining Figure. Data on steel that is not air hardenable (forged 1095 steel in this instance)

are presented in an adjoining Figure. In the tool steel example, A2 and Tl, in the form of solid blocks 50 by

100 by 100 mm (2 by 4 by 4 in.), were gas quenched with cylinder nitrogen in a vacuum furnace. Cooled gas was admitted to the chamber at 69 kPa (IO psig). As indicated in an adjoining Figure, A2 was cooled from 1010 to 345 “C (1850 to 655 “F) in 8 min. and Tl was cooled from 1290 to 345 “C (2355 to 655 “F) in I3 min. In both instances, cooling rates were suitable for maximum hardness.

Reference

I. ASM Metals Handbook, Heat Treating, Vol -k 10th ed.. ASM Intema- tional. 1991

Surface cooling curves for blocks made of types Tl and A2 tool steels quenched from austenitizing temperatures by cooled nitrogen in a vacuum furnace

Other Quenchants/Processes

Introduction

A numher of alternatives to standard quenchants/processes are availahle. including:

l Vacuum quenching l Selfquenching processes (high frequency. pulse hardening; electron

l Spray quenching l Fop quenching l Cold die quenching l Quenching in an electric or magnetic field

karn process, and laser process) l Fluidized bed quenching l l~ltrasonic quenching l HIP quenching

In addition. some processes are uniquely suited for quenching parts surface hardened in a specific process. such as quenchants for flame and induction hardened ~orkpieces.

Vacuum Quenching Parts can he quenched,in vacuum furnaces, but heat transfer rates are

relatively slob (5.7 W/m- K. or 3.6 B&h “F) in comparison with those of oil. helium, nitrogen. and air (see adjoining Table and Figure, Ref I).

Characteristics of Process Alternative quenches must be used to make vacuum quenching viable.

Other media include oil. aqueous polymers. and pas. Gas is the most commonly used. In fact. the fastest growing technology in heat treating is gas quenching in a vacuum furnace (Ref I).

Operating Information, Quenching with Gas In this procedure. the furnace is pressurized with gas after the heat

treating step is completed-this is called backfilling. The two main factors affecting quench severity are gas velocity and gas

pressure (see adjoining Figures). Gas quenching usually is done with nitrogen. argon, helium. or hydro-

gen. Physical properties of quenching gases are listed in an adjoining Table. Recently developed applications are based on gas mixtures. such as nitrogen/helium. Gas blending is a cost-elTective way of getting heat transfer rates greater than those available uith helium alone.

High pressure gas quenching (helium at a pressure of 20 bar) can produce quench severities comparable to those of conventional. recirculated oil. At very high pressures (hydrogen at SO bar) heat transfer coefficients are greater than those of eater.

Comparison of Heat Transfer Coefficients of Various Media

Quenchant Aeat transfercoefficient( W/m?. R

Ga.s.recirculated( 1000mbarN2~ Gzs.o~rrprcsrure. high \rlocib Salt bath (5.50 “C or 1070 “FJ Fluidizedbed ’

I@%I50 300400 350-450 -loo-500

Stationq oil (20-80 “C. or 70. I75 “F) Recirculated oil (30-80 “C, or 70. I75 “F) \hhter( 15-25 “C.or60-75 “FJ

100@1500 I8OO-‘100 3OOc3500

Vacuum Quenching in Oil With the exception of relatively high pressure (>20 bar) gas quenching,

quench severities with _gas are limited to those up to. but usually not including. conventional oil. k’hen lovver hardenahihty alloys are heat treated in vacuum furnaces. more quench severity is needed. which is the niche for oil.

Reference

I. Totten et al., Hmrdbook c$ Q~rtwctrunrs nnd Qtrmchitrg Technology, AS hl International. I993

Quench severities of different media

Other QuenchanWProcesses / 89

Physical Properties of Quenching Gases

Mw Nitrogen Quenching gas

Helium Hydrogen

Density,kg/m3at lS”CII bar Density ratio, v/r. to air Molar mass. kg/m01 Specific heat(a). k.lkg K Thermal conducti\ Q(a). \V/m K Dynamic viscosity(aJ, N s/m?

Ia) Gas conditions. 35 “C. I bar

1.6687 I. I 70 0.167 0.084 I I .3797 0.967 0. I38 0.0695 39.938 28.0 4.0026 2.0158 0.5204 I.o-ll 5.1931 I-I.3

177 x IO-’ 259 x IO-’ ISOOX lOA 1869x IO-’ 22.6 x IO” 17.74 x lo+ 19.68 x IO4 892 x IO”

Effect of chamber pressure at constant volumetric rate on cooling time

Effect of volumetric flow rate of gas at constant density on

Self-Quenching Processes In this category are high frequency pulse hardening (Ref I ), electron

beam, and laser processes (Ref 7). Self-quenching occurs when the cold interior of a Horkpiece is a suffi-

ciently large heat sink to quench hot surfaces hq heat conduction to the interior at a rate fast enough to allo\\ martensite to form at the surface.

Heat sources are the laser. electron beam. and inductive electric heating. In pulse hardening wjth induction heating, for example. power density is

up to about 300 W/mm-. and heat treatment is in the millisecond rangs (Ref I). With each process, areas treated are small and part size can be a restriction.

Applications Lasers. Materials hardened include plain carbon steels ( IO-IO, 1050,

1070). alloy steels (4340. X!lOO,, tool steels, and cast irons (pm!. ductile. and malleable). Examples include selectike hardening of irregularly shaped parts like camshafts and crankshafts. which are subject to wear and have fatigue-prone areas (see adjoining Figure. Ref Zj.

Electron Beam. Commonly used steels are listed in an adjoining Ta- ble. Typical parts are shown in an adjoining Figure (Ref I?).

Area treated on ductile iron camshaft is indicated (Ref 2)


Pulse Hardening. Selectively hardened parts include saws (in an ad- References joining Figure, Ref I). circular saw blades, and punching tools, plus precision engineering components for the electrical and textile industries (see Figure, Ref I). Also, this process produces very fine, acicular struc- tures that add corrosion resistance to a part.

I. G. PIBger, “HF Pulse Hardening in the Millisecond Range,” ASM Conference Proceedings, Quenching and Dbronion Control, ASM In- temational. I992

2. ASM Memls Hundbook, Hear Treoring. Vol4. 10th ed., ASM Intema- tional. 1991

Typical components heat treated with electron beam hardening method. (a) Rollerbearing element support. (b) Selected components used for both linear and rotary motion applications. Courtesy of Chemnitzer Werkzeugmaschinen GmbH (Ref 2)

Hardened teeth on band saw blade (Ref 1)

Samples of textile industry parts (Ref 1)

Steels Commonly Used in Electron Beam Hardening Applications

Material Composition, wt % AIs1 UNS No. DtN(a) C Si hln P s Cr hlo Ni v Al Cu Ti

Carbon and low alloy

-II40 GJl400 12CrMa-l 0.X3-0.45 0.17-0.37 0.50-0.80 0.03Smax 0.035 max 0.90-1.10 O.iS-0.2s 0.3Omax 0.06max 1340 Gl34OO QMnV7 0.38-0.45 0.17-0.37 1.60-1.90 0.03Smax 0.035max 0.30max O.lOmax 0.30max 0.07-0.12 ES2100 GS2986 IOOCr6 0.95-1.05 0.17-0.37 0.20-0.15 0.027 max 0.020max 1.30-1.65 0.30max 0.25

1015 G10150 c I5 0.12-0.19 0.17-0.37 0.35-0.6.5 O.O-lOmax O.O-K)ma.\ O.SOmax O.lOmax 0.3Omax IO45 G lO4SO c-is 0.12-0.50 0.17-0.37 O.SO-0.80 0.04Omax O.O-!Omax OSOmax O.lOmax 0.30max I070 Gl0700 Ck67 0.65-0.72 0.15-0.50 0.60-0.80 0.03Smax 0.035 max 0.3Smas 0.35 mar __. 0.35

55CrI 0.52-0.60 0.17-0.37 0.5-0.8 0.03.5 max 0.2-0.5 0.3max 0.02-0.0s 0.3 max 0.015

SOCrV 1 0.37.0.55 O.-l max 0.7-I. I 0.035 0.03 max 0.9. I .2 0. I-O.2

Tool steels

02 WI

T31.502 9OMnV8 0.8.5-0.9.5 0.15-0.35 I 80-2.00 0.030max 0.030max .._ 0.07-0.12

T72301 c IOIIWI 0.95-1.04 0.15-0.30 0.15-0.25 0.02Omax 0.020mai 0.20max 0.20maxfb)

(a) Deuwhe Industrie-Normen. (bj0.25 max Cu

Other Quenchants/Processes / 91

Fluidized Bed Quenching Nitrogen and air are common quenching media. Other quenchants are

argon, carbon dioxide, helium, or hydrogen. Critical variables are rate of fluidizing gas flow and thermal conductivity of the gas.

Characteristics of Process

Control over quenching with this process CompXes favorably with that

of other liquid quenchants. The heat transfer mechanism is uniform throughout the entire temperature range. and is dominated by the properties of the gas phase. Quench rates are reproducible, do not degrade with time.

and can be adjusted within wide limits and operate over a wide temperature range (Ref I ).

The quenching or heat treating rate can be adjusted by altering operating conditions of the fluid bed. Variables include particle size and volume (aluminum oxide is preferred). rate of fluidizing gas flow, and the thermal

conductivity of the gas. Nitrogen usually is the choice. Quench seberitics are between those of still air and slow air.

Comparisons of quenching rates for fluidized beds operating on nitrogen, and vacuum furnaces operating at 2 bar and 6 bar quench pressures at temperatures up to 575 “C (1065 o F)

In comparison with other heat treating/quenching processes, the fluid bed is less sensitike to load densit) and part geometry. Because of the liquidlike characteristics of the fluid bed. parts are surrounded by aluminum oxide particles. and the high heat capacity of aluminum oxide does not require complete gas flow over all surfaces because heat is removed by conduction.

Operating Information In treating H-I I and H- 13 forging dies the following procedure was used

(Ref I):

l Preheat work to 595 “C ( I I05 “Fi l Austenitize at IO-10 “C (I905 “F) l Step quench at 595 “C (I IO5 “F) 9 Fluid bed quench at ambient temperature for 5 to 7 min at approximately

290 “C (555 “F) l Air cool to room temperature

A second, two-stage process: quenching in helium first, followed by quenching in nitrogen. Application: austempering 4340. medium carbon steel tools. replacing salt processing. i.e., austenitizing at 920 “C (1690 “F) and quenching into salt at 330 “C (610 “F). then holding for 30 min. For the two-stage process. the quench temperature was reduced from 330 “C (625 “F) to 295 “C (565 “F).

Step NO. 1: Short pulse (30 to 60 s) of helium to drive the load past the nose of the cooling curve. (In treating 4340, the fust IO s of the cooling curve is critical). Beyond this point allowable time is increased.

Step NO. 2: Gas is switched from helium to nitrogen for the remainder of the cycle.

In cases u here hardness values in fluidized bed quenching are slightly lower than those of the other quenchants, higher hardness can be obtained by slightly decreasing fluidized bed temperatures.

Range of Applications

Standard and special fluid bed processes are available. Air hardening tool steels. for example, are within the range of the former, while medium and low allo) steels are among the applications of the latter.

Standard Process. Performance of this process in treating air hardening tool steels is said to compare favorably with that of high pressure gas quenching in a vacuum furnace. In this instance. two critical factors are: the quench rate must be severe enough to effect full metallurgical transfor-

mation of thick sections. Chile not causing severe distortion or cracking.

COmpariSOn of results in quenching and normalizing with: A, salt solution; B, agitated water; C, still water; D, oil; E, fluidized bed; F, normalized. Specimens were 12.6 mm (0.50 in.) diameter bars.


Quenching rates for the fluid bed process and those for high pressure gas quenching in vacuum are compared in an adjoining Figure. The fluid bed

rate is slightly higher than that at 6 bar quench pressures. The material is M-2 high speed tool steel. Other comparisons are made in a second Figure.

Special Process. The standard fluidized bed has insuffwient heat transfer characteristics to be useful in quenching medium to IOU allo] steels because the critical portion of their cooling cycle is the first IO s. which precludes the application of a number of these alloys in austemper-

ing. marquenching, and direct hardening. This limitation is bypassed by modifications in the standard process. Quenching speeds are significantly higher.

The special process has t&o phases. In the first phase, helium replaces nitrogen for cooling in the critical

portion of the cycle (the nose of the isothermal transformation cycle).

Helium has a gas conductivity nearly six times that of nitrogen. and the fluidization rate is doubled. The helium phase takes 30 to 60 s.

In the second phase, nitrogen replaces helium for the rest of the cycle.

Example of an Application: nustempering 4310 steel tools. In salt

processing. parts were austenitized at 920 ‘C (1690 “F). and they \ierc quenched in salt at 315 “C (600 “F). then held at that temperature for 30 min. In processing uith the modified fluid bed process. the austemperinp quenching temperature was reduced from 3 I5 to 295 “C (600 to 565 “F). Hardness of these parts was lower than that of those treated ~rith the salt

process. The desired result H;LS obtained by slight reductions in the fluid bed temperature.

Steel parts treated by this process and prior quenching techniques include the foIloH ing:

52 100, bearing races. oil quenched 1340, wood routing bits, austempered at 350 “C (660 “F) Modified S-3. screw driver bits austempered at 3 I5 “C (600 “F) O-l, general tooling. marquenched in salt at 210 “C (410 “F) Ductile iron. crankshaft. oil quenched 86B-lO. forging, oil quenched 5 150, machined parts. oil quenched

Reference

I. A. Dinunsi, “AdLances in Fluidired Bed Quenching,” p 71. ASM Conference Proceedings. Qutvrchittg md Disronion Conrtd, ASM In- ternational. I992

Other References

l Totten et al., Hmdbook oj Qrret~chatrrs md Qwm-hittg Technology AShl International

l .4W Aie~l~ls Hntdbook. Hear Twctrittg. Vol 4. 10th ed.. ASM Intema- tional

Ultrasonic Quenching Virtually any liquid quenching medium can be used in ultrasonic quenching. Reference

Characteristics of Process Vapor blanket formation is readil) interrupted by ultrasonic energ! (see

Figure).

I. Totten et al.. Hmcibook of Quettchcwts and Quenching Technology, AShl International. 1993

Operating Information Ultrasonic agitation substantially increases quench severit) (see Table).

but the cracking and distortion that can be caused b>, oil, Water, or brine quenchants often are eliminated. Reductions in distortlon and cracking are often accompanied by an increase in hardness.

Comparison of vapor blanket phases during oil quenching and ultrasonic quenching

Grossmann H Values of Various Quenchants with and without Ultrasonic Energy

Quenchant

Oil

Still quench Violent agitalion llltra.sonic agitation

Brine

Still quench Violenr agitation Uluasonic agitalion

Hot salt at (400 OF)

Still quench Violenl agitation llluasonic agifation

Gmssmann H value

0.2YO.30 0.80/1.10

I .bS

2.0 s.0 7.5

0.30 I.20 1.80


HIP Quenching The HIPquencher is an offshoot of the more familiar hot isostatic press

used to densify metal powders and ceramics and to improve certain properties of castings.

Gas (usually argon) is the only cooling medium in HIPquenching. Rapid cooling is obtained with high pressure gas at 800 to 1800 bar. Heated gas is cooled in a heat exchanger. Gas pressure pushes gas atoms closer together, increasing the number of atoms that remove heat from steel surfaces. The heat exchanger is located in the HIP vessel outside the hot zone.

Characteristics of Process The extremely high heat transfer coefficient of gas under high pressure

makes for less variation in temperature in different areas of a part. reducing the likelihood of distortion. Characteristics of several different quenching methods are compared in an adjoining Table. The heat transfer coefficient of the process is of the same magnitude as that for the fluidized bed and about three times greater than that for the vacuum furnace (see Figure). Gas temperature is closely controlled by computer as a function of time.

Applications Higher hardenability steels such as high speed steels and other tool steels

are in the applications range of the process. Densiftcation and heat treatment can be combined in a single operation.

References

I. Bergman and Segerberg. “HIP Quencher for Efftcient and Uniform Quenching.” and Segerberg. ASM Conference Proceedings, Qumching and Disioniotr Cotlrrd. AShl International, I992

1. A. Traff. M~!nl Powder Repon, 15 (199Oj, 1 3. ltdus~rial Qutwchitrg Oils-Dt~rennitlnrion of Coolitrg Ctrarac-

reristics-Lahomroy T&r Mrrhod. Draft international standard ISO/DIS 9950. International Organization for Standardization (submit- ted 1988).

1. S. Segerberg, n/F‘-mpporr 920-71. n/F, Giiteborg, Sweden

Characteristics of Four Quenching Methods

Method of Temperature of quenching quenchant, OC Gas

PMSU~, bar

Vacuum Sah halh Fluidizrd bzd HIPquencher

Refl

60 Nitrogen 230 20 Nitrogen

Drlcrrasing from 1000 Argon

Heat transfer coefficients of different quenching methods (Ref 1)

Spray Quenching Process High pressure streams of quenching fluids arc directed onto areas of a

uorkpiece requiring higher cooling rates. Quenchant droplets formed by the spray account for the speedup in cooling rate. Lou pressure spraying. providing a flood-type now, is preferred in quenching with some aqueous

polymers. Spray nozzles are located on a quench rig. Quenching characteristics can be innuenced by volume and pressure of the quenchant. as well as the design of spray nozzles.


Fog Quenching Process A tine fog or mist of liquid droplets in a gas carrier is the cooling agent.

Cooling rates are lower than those in spray quenching because of the relatively low liquid content of the stream.

Cold Die Quenching Llsed for parts such as thin disks and long. slender rods that distort

excessively when quenched in conventional liquid media. Quenching is between various forms of cold, flat, or shaped dies, which usually are in a press close to austenitizing operations.

Reference

I. ASM Mrrals Haruibook, Hear Treating, Vol 4. 10th ed., ASM fntema- tional. I99 I

Quenching in an Electric or Magnetic Field

In quenching steels in an electrical field, electrical current is passed through the part while it is submerged in a liquid such as oil or water.

In quenching in a magnetic field. steel is quenched into an aqueous suspension of IO nm magnetic particles.

Characteristics of Process In quenching in an electrical field, uniformity of surface heat transfer is

enhanced by destabilizing vapor blanket cooling by passing an electrical current through the workpiece.

Operating Information It has been demonstrated. for example that the hardness of a 0.15 percent

carbon steel can be increased IO to I8 HRC, while quench-induced microstresses are virtually eliminated (see Figure).

Similar results have been obtained in magnetic field quenching. Cooling rates throughout the quench can be controlled by the concentration of magnetite particles and the force of the magnetic field (Ref I .4).

References

I. Totten et al., Handbook qf Q~tetrchatm and Qttenctring Techtwlog?; ASM International, I993

2. A.A. Skimbov. LA. Kozhukhar. and N.N. Morar. So\: Eng. Ai~p/. Nec- rrochem, Vol 2, 1989, p 136-138

3. A.A. Skimbov, LA. Kozhukhar. and N.N. Morar. Elekrrontray Obrab. MareI. Vol2, 1989. p 87-88

1. S.N. Verkhovskii. L.I. Mirkin. and A.Ya. Simonovskii. Fi:. Khitn. Obrab. Mares:, Vol2. 1990, p I27- I32

Microstresses on perimeter of 0.45 percent carbon steel quenched in different media (Ref 1)


Quenching Flame and Induction Hardened Parts Water is the common quenchant for flame and induction hardened

workpieces. Other media are oil, soluble oil, compressed air. aqueous polymer solutions. and brine (Ref I).

Reference

I. ASM Metals Handbook, Hear Treating. Vol 1. 10th ed., ASM Interna- tional, 1991

Characteristics of Process Water is the choice unless metallurgical considerations call for less

severe quenching media.

Operating Information

Recommended Orifice Sizes and Fluid Pressures for Induction Spray Systems

Open and submerged spray systems generally are used in conjunction with induction hardening. Spray oriftces for water quenching are relatively

small to maximize cooling rates. Different orifice sizes and spray pressures are required in quenching with aqueous polymers. High pressure and fine

spray cause premature rupture of polymer films on hot metal surfaces, which reduces cooling rates. Recommended orifice sizes and fluid pressure for quenching steels from an austenitizing temperature of 845 “C (I 555 “F) with polyethylene glycol (PAG) are listed in an adjoining Table (Ref I).

Orifice -m(b) diameter

Tjpe ofsprq (8) kPa psi mm in.

Q-=n <l-ICI <X 3.2 ‘4 Submerged >ZlS >-Ml 6.1 ‘4

(a) All of the cooling c‘urves for the quench factor correlation were determined using AlSl bpe 30.4 stainless steel prohes. thy Data for LJCON (Union Carbide Chemicals and Plastics Company. Ins J Quenchant B

Documents

Steel Quenching Technology.pdf