HEAT TREATING PROGRESS • SEPTEMBER 2009 45

A vacuum furnace and control systems developedfor nitriding yields uniform,precision nitrided components in reducedcycle times with controlledwhite layer to customerspecifications.

Donald Jordan* Solar Atmospheres Inc.Souderton, Pa.

*Member of ASM International and member,ASM Heat Treating Society

urrently, nitriding is carriedout predominantly in pit typevertical furnaces with metalalloy retorts to hold the workload during the nitriding

cycle. The large thermal mass of thesefurnaces requires long heat-up andcool-down times. Another factor isthat the ammonia nitriding gas cracksnot only on the work load but on themetal retort too. In time, this leads tonon-uniform nitriding of the work,and the retort has to be conditionedbefore uniform nitriding can be re-stored. In contrast, the much smallerthermal mass inherent in vacuum fur-naces (as well as other features) offersan opportunity of designing a moredesirable vacuum furnace for pro-viding efficient uniform nitriding.Such a furnace was designed and de-veloped over several years to replacetraditional retort gas nitriding[1].

Furnace ConsiderationsTwo of the main factors in con-

structing a vacuum furnace system fornitriding are the physical design of thefurnace proper and the nitriding at-mosphere control system. The furnace

that was developed is a horizontal,single-chamber, front-loading furnaceequipped to case harden a stationaryload of various materials by means ofnitriding gas or gas mixtures duringthe heat cycle. After nitriding, the ma-terial is cooled rapidly by external gascooling at a positive pressure in asingle chamber of the furnace (Fig. 1).The chamber is separated into an innerportion and an outer portion.

The inner portion (work chamber)consists of graphite insulation, eightvertically arranged heating elements,and a number of strategically placedgraphite baffle plates. Piston-drivengraphite port plugs are closed duringthe nitriding cycle, providing a gasseal, and are opened at the end of thenitriding cycle to provide rapid and ef-ficient cooling. Two graphite fans atthe top of the inner portion circulatethe nitriding atmosphere uniformlywithin the within the work chamber.This system provides a uniform distri-bution of the nitriding gas, and thusproduces uniform case hardening ofthe work load (Fig. 2).

The hot zone includes a work zoneof approximately 36 in. wide × 30 in.

C

VACUUM GAS-NITRIDING FURNACE PRODUCES PRECISION NITRIDED PARTS

Fig. 1 — Vacuum furnace and control system.

high × 48 in. deep. It is made com-pletely of graphitic materials that areinert to the anhydrous ammonia usedin nitriding.

The furnace provides rapid, uniform

convection heating and rapid cooldown. Thus, the cycle time is reducedsignificantly over retort-type nitriding.However, in addition to an efficientlydesigned vacuum furnace, it is neces-

sary to have an atmosphere controlsystem that provides precise control ofthe nitriding atmosphere. The controlsystem was supplied by Super Sys-tems Inc. (Cincinnati, Ohio).

Precise control of the nitriding at-mosphere is a prerequisite to pro-viding the required microstructure inthe nitrided work load. The structureis dependent on the degree of disso-ciation (VN2 + H2)[2] of the ammoniaor a factor called the nitriding poten-tial (Kn)[3].

The possible phases that can bepresent in the nitrided layer includealpha (�) phase (a solid solution of ni-trogen in ferrite), gamma-prime (�’)Fe4N, and epsilon (�) Fe2-3N. Some orall of these phases may be present. Astructure at the surface (white-layer)can be composed of �’ and/or �. Con-trolling the ammonia concentration byflow or by diluting with N2 or H2 willdictate the degree of dissociation, and,in turn, the phases that will be present(Figs. 3 and 4). The Kn value can be cal-culated using the following formulas:For ammonia-nitrogen mixtures:

Kn = 1 - BR(+N2) {1.5(BR - N2o)/(2 - N2o)}1.5 (1)

For ammonia-hydrogen mixtures:

Kn 1 - BR(+H2) ={BR(1.5 - H2o) + 0.5H2/(2 – H2o)}1.5

(2)

where BR is the burette reading, N2o

or H2o is the amount of nitrogen orhydrogen, respectively, added origi-nally, and H2 is the hydrogen fromdissociation.

The values of Kn calculated fromequations (1) or (2) in conjunction withthe Leher Diagram (Fig. 5) may be usedto determine the structures that will bepresent during nitriding with am-monia mixtures[4]. The critical valuesof Kn from the Leher Diagram for thephase boundaries versus temperatureat one atmosphere are shown below.

Temp., oF Kn (� - �’) Kn (�’ - �) 925 0.30 2.2 975 0.23 1.5 1000 0.21 1.3 1025 0.20 1.2 1050 0.16 1.1

There are various automated instru-ments used to determine the degree ofdissociation. Measuring the percent ofhydrogen works well if nitriding usingpure ammonia or a mixture of am-

46 HEAT TREATING PROGRESS • SEPTEMBER 2009

Fig. 2 — Schematic of furnace hot zone and vacuum chamber.

Fig. 3 — Dissociation curves of NH3 + N2 or H2.

Fig. 4 — Possible nitrided phases for a 20% NH3 + N2 mixture at 1 atmosphere.

Cooling ggas oout CCooling ggas iin

Graphite fan

wheels

Port plugs

Heating elements

0 110 220 330 440 550 660 770 880Percent aammonia

100

80

60

40

20

0

Per

cent

di

ssoc

iatio

n N2

H2

400 5500 6600 7700 8800 9900 11000 11100Temperature, °°F

100

95

90

85

80

Deg

ree

oof dd

isso

ciat

ion

(bur

ette

rread

ing)

�

�’

�

monia and nitrogen. A good check onthe automated degree of dissociationinstrument is can be performed usinga nitriding manual burette (Fig. 6)[5].

Nitriding CycleAtypical nitride cycle includes start

up, nitride, and cooling the workload.Start-up steps are:

1. Place the workload directly in thefurnace or in alloy steel baskets thatare placed on graphite hearth rails.

2. Close the furnace door and gasport plugs to seal the furnace fromleakage of gas.

3. Evacuate furnace outer chamberand hot zone to approximately 10-2 torrto remove essentially all of the air fromthe furnace.

4. Backfill furnace with nitrogen toapproximately to 0.5 psig (800 torr) viaa back fill valve.

5. Introduce partial pressure of ni-trogen through a gas inlet, and startgas circulating fans.

6. Heat furnace to set nitriding tem-perature (up to 1400°F).

7. Pump out a portion of the ni-trogen at the set temperature to a set

pressure below 800 torr.8. Backfill furnace with ammonia to

a set furnace pressure of 800 torr.9. Introduce partial pressures of am-

monia and nitrogen continuously at afixed ratio with flow controllers, whichare set to control the flow as requiredby the specification. Gas is vented toatmosphere via a gas purge valvingarrangement.

The gas is circulated and is also fedinto a nitriding gas analyzer that de-termines the hydrogen concentration.The results are fed back into the con-trol system that regulates the gas flowto the desired ratio.

Cooling the workload involves:1. Shut off heat and ammonia flow,

and pump down furnace to about 1torr.

2.Backfill furnace with nitrogenwhen pressure of 1 torr is achieved to apressure of about 1,010 torr.

3. Automatically, a mechanism opensgas port plugs, and the blower and cir-culating fans are turned on to gas coolthe workload. The gas is circulatedthrough an external cooling heat ex-changer, where the cooled gas is recir-

HEAT TREATING PROGRESS • SEPTEMBER 2009 47

Fig. 5 — Lehrer diagram

Fig. 6 — Dissociation burette schematic for ammonia.

dNH3

expanded Dissociated = 22�������������������������������Dissociation

NH3 = ����������������������������������������������������������rate == dd == HH2 = hh == 00.75d dd2� / ((1 ++ �)

N2 = nn == 00.25dNH3 = AA AA’ == AA - �����������������������������r == AA - dd ==

A - � / (1 ++ �) rr

Inlet aatmosphere AAfter ddissociation NNormalized ooutlet atmosphere ((vol == 11) BBurette

logic

0.01 00.1 11 110Nitriding ppotential

1000

900

800

700

600

Tem

pera

ture

, KK

Ammonia ccontent, vvol% NNH3 in HH2 @ 11 aatm1 33 110 330 660 880

800

700

600

500

400

300

Tem

pera

ture

, °°C

��

�

��� ’

culated to the furnace to further cool theworkload to the desired temperature.

Typically, using this vacuum furnace,cycle time can be reduced by up to 50%of the time required using conventionalretort-type furnaces. Furthermore, a va-riety of materials have been nitridedsuccessfully to customers’ specifications;including 4140, 4150, 4330, and 4340steels, Nitralloy, and H-11 and A-2 toolsteels. Figure 7 shows representative

parts ready for nitriding, and Fig.

8 shows a crosssection of a

nitrided case.

ConclusionThe develop-

ment of this fur-nace and control

systems allows the company to provide nitriding services that yielduniform, precision nitrided compo-nents with controlled white layer to customer specifications. Singlestage, and two stage (Floe) processinghave been developed that providesuperior metallurgical properties

more efficiently, for virtually any application. HTP

References1. U.S. Patent application dated December13, 20092. V.M. Zinchenko, et. al., Nitrogen Poten-tial, 3rd Lakhtin Memorial Lecture, 9thSeminar of the Intl. Fed. for Heat Treatmentand Surf. Engrg., 21-23 Warsaw, Poland,Sept. 2003.3. D. Jordan, et. al., Low Torr RangeVacuum Nitriding of 4140 Steel, HeatTreating Progress, (8), p 33-38, Mar./April,2008. 4. E. Lehrer, Z Electrochem, Vol. 36, p 383-392, 1930.5. J. Darbellay, Gas Nitriding: An IndustrialPerspective, MSE 701 Seminar, March 22,2006.

Acknowledgement: The author acknowl-edges the contributions from Solar Atmos-pheres Research and Development teamincluding Harry Antes, Ph.D., technicalconsultant; Virginia Osterman, Ph.D., tech-nical consultant; Trevor Jones, project engineer; and Brittney Dhein, technicalanalyst.

For more information: Don Jordan, VicePresident/Corporate Metallurgist, SolarAtmospheres Inc., 1969 Clearview Rd.,Souderton, PA18964; e-mail: dfj@solaratm.com; www.solaratm.com.

48 HEAT TREATING PROGRESS • SEPTEMBER 2009

Fig. 7 — Representative parts for vacuum gas nitriding.

Fig. 8 — Cross section of vacuum gas-nitrided surface.