Unit 3Stainless and tool steels

Stainless steelsUsed for corrosion and heat resistant applications

Contain large amounts of chromium

12% Cr raises the critical temperatures and reduces the austenite region. With sufficient amounts of carbon, these steels can be heat treated to a martensitic structure.

The response to heat teatment depends upon their composition.

Indicated by three digit numbers like 304, 402 etc.

2xx-Cr-Ni-Mn - non hardenable -austenitic - non magnetic

3xx- Cr-Ni - non hardenable -austenitic - non magnetic4xx- Cr - hardenable - martensitic - magnetic4xx-Cr -non hardenable -ferritic -magnetic5xx-Cr - low chromium-heat resisting

Last two digits are serial numbers to designate a variety of that group.

Martensitic Stainless steels

Straight chromium steels with 11.5 to 18% Cr.C 0.15Mn 1.25 Si 1

For turbine blades and corrosion resistant applicationsMagneticCan be machined (poorer machinability than plain carbon steels. Machinability can be improved by adding small amounts of Selenium or Sulphur.)

Hot working possible.

Can be hardened (by air cooling or oil quenching itself)

Ferritic Stainless steels

14 to 27% CrLow in carbon but high in Cr compared to martensitic steels.Not hardened by heat treatment.Only moderately hardened by cold working

Can be cold or hot worked.Achieves maximum softness in annealed condition.

As annealed, their strength is 50% higher than plain carbon steels andcorrosion resistance and machinability is better than martensitic steels.

Annealing is done to relieve stresses due to welding or cold working.Susceptible to embrittlement during slow cooling during annealing.Since martensite is not formed and since there is embrittlement possibility, these steels are not tempered.

Austenitic Stainless steels

Chrome-nickel or chrome -nickel- manganese alloysAustenitic, non magnetic and do not harden by heat treatment.

Total content of nickel and chromium is at least 23%

Difficult to machine. Can be improved by Selenium of sulfur additions.

Best high temperature strength and reistance to scaling.Hence the best corrosion resistance.

Cold working causes work hardening.Can be hot worked easily.Type 302 stainless steel is more used.(austenitic)Modified into 22 different alloys.

Lowering the carbon to 0.08% gives stainless steel type 304 with improved weldability. Used for most fabrication that needs welding.

Tool steels

Steels used to make tools and dies.

Tools are used for cutting or forming purposes-to shape other materials.

Classified based on

1. Quenching method air hardening, oil hardening, water hardening

2.Carbon contentCarbon tool steels,low alloy tool stels or medium alloy tool steels

Tool steelsMethod followed by AISI (American Iron and Steel Institute)

W- Work hardeningS- Shock resisting

O-Oil hardening (cold work Steel)A-air hardening (cold work Steel)D-High carbon high chromium(cold work Steel)

H-Hot work tool steel

T-Tungsten base HSS (high speed steel)M-Molybdenum base HSS

Tool steels-properties needed

1.Must be hardenable

Definite temperature and definite cooling rates (known to user) for hardening

2.Resistance to decarburisation3.Resistance to softening on heating (hot hardness)4.Must withstand high temperatures during operation (heat resistance)5.Must withstand high wear environment (wear resistance)6.Must withstand impact load (toughness)7.Possible to shape (machinability)

Selection of Tool steels

The type of operation decides what property is required in the tool. Accordingly, the composition of the tool steel is chosen.

Cutting- hardness, heat resistance and wear resistanceLathe cutting tool, milling cutters

Shearing-hardness and toughness (resistance to fracture)Punches and shears

Forming-high strength, impact strength, hot hardness (not losing its strength at hot condition)Punches and rolls for shaping hot steel sheets

Drawing and extrusion-toughness, wear resistance and hot hardness.Wire drawing dies

Some popular tool steels

The Joint Industry Conference JIC, USA uses the following symbols

T- tungsten based high speed steelM-Molybdenum based HSSD-HCHCr (High Carbon High Chromium)A-Air hardeningO-Oil hardeningW-Water hardeningH-Hot work steelS-Shock resisting steel

W8 means a water hardening steel with 0.8%C.

HSSTungsten base

T1 C 0.7, Cr 4, V1, W 18

T4 C 0.75, Cr 4, V1, W 18, Cobalt 5 %

T6 C 0.8, Cr 4.5, V1.5, W 20, Cobalt 12

Molybdenum based

M1 C 0.8, Cr 4, V1, W 1.5, Mo 8M6 C 0.8, Cr 4, V1.5, W 4, Mo 5, Co 12

(High speed-more heat-more heat resisting elements)Carbon less than 0.8% eutectoid composition

HCHCrD2 C 1.5, Cr12, Mo 1D5 C 1.5, Cr12, Mo 1, Co 3D7 C2.35, Cr12, Mo 1, V 4

Note: carbon more than 0.8% hence high in carbon

Air, oil and water hardeningAir hardeningA2 C 1, Cr5, Mo 1A9 C 0.5, Cr5, Mo 1.4 Ni 1.5, V 1

Note: carbon closer to the eutectoid compositionCarbide formers are less than in HSS and HCHCr

Oil hardeningO1 C 0.9, Mn 1, Cr 0.5, W 0.5Note the presence of Mn which renders the steel more hardenable

Water hardeningW2 C1.4, V 0.25W5 C1.1, Cr 0.5

Precipitation hardening steel

17-7 PH

Cr17%, Ni7%, Si 0.4, Mn0.7 and C 0.07

Soultion treated in roll mill and supplied .

After forming to required shape, they are aged to attain the required increase in hardness and strength.

Precipitation hardening17-7PH is solution annealed at 1950 deg F ,followed by air cooling.

This produces austenite with around 20% delta ferrite.In this condition, the alloy is soft and can be easily formed.

In Temper hard (TH) sequence,austenite is conditioned by reheating to 1400 deg F.

This will precipitate the chromium carbides, reducing the carbon and chromium content of the austenite.Therefore transformations are possible while further cooling.

Cooling is continued till 60 deg F to obtain the necessary amount of martensite.

Aging is carried out at 1050deg F.

Maraging steels

A series of iron base alloys capable of attaining

yield strengths upto 300,000 psi. in combination with excellent fracture toughness.

Low carbon (0.03%),18-25 % Nickel and other hardening elements.

Yield strength: The stress at which a material exhibits a specified deviation from proportionality of stress and strain.

Fracture toughness:resistance to crack propagation.

Maraging steels

As annealed, these steels are martensitic.

They achieve high strength on being aged in the annealed or martensitic condition.

This martensite is soft and tough as compared to the hard and brittle martensite formed in conventional low alloy steels.

This ductile martensite has a low work hardening rate and so can be cold worked to a high degree.

Maraging steels

There are two groups, based on hardening element used.

1.The 18% nickel grades use cobalt molybdenum additions

2.The 20% nickel grades use titanium-aluminium-columbium additions.

Maraging steels

18% NickelHeated to 1500 deg F , held for 1 hour .

Soaked to anneal austenite and dissolve hardening elements Co and Mo.

Air cooled to 100 deg F, forms martensite of 28 – 32 RC

Reheated to 900 deg F, held for 3 hrs- aged to harden.

stress relief also occurs at the same time.

Hardness of 52 RC can be achieved. 300300300

300300

3 hrs

1 hr

300

Deg F

900

1500

Ms

10028 RC 52 RC

Maraging steels

18% Nickel

•The interest is in achieving high strength at room temperature.

•Simple heat treatment carried out at moderate temperature is enough to achieve good properties.

•Section size, heating and cooling rates are not important.

•Very low in carbon content and so no problem of decarburization.

•Protective atmosphere is not required.

•Low aging temperatures means less distortion.

•So,no deformation on hardening and not much machining is required after heat treatment.

Maraging steelsEffect of additives on maraging strength development

Solution treated 110,000 psiAfter maraging 300,000 psi

Iron nickel martensite 25Rc

A weak response to maraging is seen after addition of 7% cobalt.

The addition of molybdenum alone gives a slight increase in annealed hardness and good maraging response.

When Mo is added in presence of 7% Cobalt, an increase in hardness greater than the combined effect of both the elements is seen.

Co

Co7Co+Mo

Mo

Co

Solution treated

24

52

Rockwell C

2 4 6 8

%Mo or %Co

Maraging steels

25%Nickel

Largely austenitic after annealing.

The conversion to martensite is done by ausaging or cold working.

Ausaging-conditioning treatment at 1300 deg F.Reduces the stability of austenite by causing nickel titanium compounds to precipitate from the austenitic solid solution.

It raises the Ms temperature so that martensite will start forming at room temperature.

Cold worked to 25% to start the transformation to martensite and is completed by refrigeration at minus100 deg F.

Maraging steels

20% NickelLower alloy content Freedom from cobalt and molybdenum. Useful in certain environments and applications.

Compared to the 25% grade, this does not require a conditioning treatment to become martensite.Ms temperature is above room temperature.

Disadvantages:

Lower intoughness, resistance to stress corrosion cracking and in dimensional stability during heat treatment.

Maraging steels

Applications for maraging steels

Hulls for hydrospace vehiclesPressure vehicles

Motor cases for missilesMortar and rifle tubing

Hot extrusion dies

Low temperature structural parts

Cold headed bolts

(Complex shapes that need to be strong after shaping)

Solution treatment

Heating an alloy to a suitable temperature

Holding at that temperature long enough to allow one or more constituents into solid solution

Then cooling rapidly to hold the constituents in solution.

The alloy is in a supersaturated, unstable state.

Age Hardening

Aging:

A change in properties in alloys, that occurs slowly at room temperature and more rapidly at high temperatures.

Age hardening:

Hardening after aging, usually after rapid cooling or cold working.

Cold working and Hot working

Cold working:

Deforming the metal plastically at a temperature lower than the recrystallisation temperature.

Hot working:

Deforming a metal at such a temperature and rate that strain hardening does not occur . The low limit of temperature is the recrystallization temperature.

Strain Hardening

Strain hardening:

An increase in hardness and strength caused by plastic deformation at temperatures lower than the recrystallisation temperature.

Recrystallization

Recrystallization temperature:

The approximate minimum temperature at which complete recrystallization of a highly cold worked metal occurs within a specified time, usually one hour.

Recrystallisation:

The formation of new strain free grain structure from that existing in a cold worked metal, usually accomplished by heating.