The development of processsensors and computer control systems has proven to be valuable for nitridingprograms, improving both nitriding and nitrocarburizing process consistency and repeatability.

Patrick Weymer**Ipsen Inc.Cherry Valley, Ill.

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

n the past, gas nitriding and ni-trocarburizing processes havebeen controlled using empiricalcalculations without any atmos-phere process control. The gen-

erally accepted procedure to controlthe nitriding process was to adjustthe flow rate of ammonia gas andother possible gas additions (en-dothermic gas, nitrogen, and CO2),and occasionally to use an Orsat de-vice to measure the amount of am-monia exhausted from the furnace.This procedure, for the most part, ledto reproducible nitriding results, es-pecially in processes using constantatmosphere flow rates. However, inpure nitriding and nitrocarburizingprocesses, deviations in the resultingpart microstructure sometimes oc-curred for unknown reasons. Thecase depth on parts could be shallowin one load and could meet require-ments in preceding and subsequentloads.

For this reason, work began tenyears ago on the development of a re-liable, continuous process controlsystem capable of monitoring devi-ations in the gas atmosphere of thefurnace.

In both nitriding and nitrocarbur-izing, atomic nitrogen forms via a cat-alytic decomposition of ammonia(NH3) on the metal surface of a work

piece. This decomposition obeys thefollowing reaction equation:

NH3 —> [N] + 3/2 H2 [1]

Nitriding reaction: This ammonia de-composition also takes place in thefurnace atmosphere, only here it ismolecular nitrogen that is formed,not atomic

NH3 —> 1/2 N + 3/2 H2 [2]

Atmosphere reaction: Different ironnitrides will form in the white layerdepending on the amount of atomicnitrogen that is available to a part. Atthe heat treatment temperature fornitriding of approximately 975°F(525°C) and at a nitrogen concentra-tion between 5.7 and 6.1%, thegamma prime (γ’) cubic face-centerednitride (Fe4N) is formed, while theepsilon (ε) hexagonally densest ni-tride (Fe2-3 N) will form at a highernitrogen content of between 7.8 and11.3%.

However, while the concentrationof atomic nitrogen can be used to de-termine the composition of the whitelayer, the so-called nitriding activityor nitriding potential (Kn) of the fur-nace atmosphere can also be used forthis purpose. The higher the nitridingpotential, the more nitrogen can bemade available to the parts and thehigher the nitriding effect.

Kn is the quotient of the partialpressure of the ammonia still presentin the furnace (the amount of am-monia that has not yet dissociated)and the partial pressure of the hy-drogen (H2) that has already formedby ammonia dissociation. Kn is cal-culated as follows:

Kn = p(NH3)/p(H2)3/2 [3]

The exponent in the denominator isdue to factors from the chemical re-action equation. If we now take Kn asa value for nitride forming, we ob-tain the diagram developed byLehrer (Fig. 1). Although the diagram

CLOSE PROCESSCONTROL

HEAT TREATING PROGRESS • MARCH/APRIL 2008 51

IYIELDS NITRIDINGCONSISTENCY, REPEATABILITY

Fig. 1 — Lehrer diagram for pure iron.

450 500 550 600 650 700Temperature, °C

2

1

0

-1

-2

-3

-4

lg p

(HN

3)2/

p(H

2)3

10

1

0.1

0.01

Kn

= p

NH

3/p

H2)

3/2

εε

γγ ’

ααγγ

%[N]

0.5 0.0 5.5 6.0 7.5

only applies to pure iron, it can be used as a good approximation for steels that are not too highly alloyed.

As can be seen in the Kn potentialequation, the data required to calcu-late Kn includes:

• Quantity and composition of in-coming process gases (mass flowcontrollers can be used to measurethe incoming amount of each gas)

• Furnace pressure (a pressuresensor can be used)

• Hydrogen content present in thefurnace (Ipsen’s HydroNit sensor canbe installed in a furnace to provide anin-situ measurement of the hydrogen)

Ammonia is the only source ofhydrogen when using only nitrogenand ammonia during nitriding.Thus, the measured hydrogen con-tent can be calculated from theamount of ammonia that has de-composed. Subtracting the amountof ammonia gas in the furnace fromthe amount of incoming ammoniagas leaves the amount of ammoniathat is still available for nitriding.This provides the data about thecurrent composition of the gas inthe furnace; that is, the ammonia re-maining in the furnace still avail-able for nitriding and the molecularhydrogen and nitrogen formed by

the ammonia decomposition. Thevolume percentages and the in-ternal furnace pressure are thenused to calculate the partial pres-sure of the individual gases partialpressure required to calculate Kn.

If the incoming gas consists of bothammonia and hydrogen, then theamount of incoming hydrogen mustalso be deducted from the hydrogenmeasured by the HydroNit sensorprior to calculating the amount ofammonia still available for the reac-tion. The amount of nitrogen flowinginto the furnace must also be takeninto account when determining thepartial pressures of the individualgases in the furnace.

Equation [3] can now be used tocalculate the Kn at any time duringthe process from the calculated par-tial pressures for hydrogen and am-monia. Knowing the current hy-drogen level in the retort isabsolutely essential for determiningKn. The HydroNit sensor continu-ally makes such a measurementpossible directly in the furnace at-mosphere (Fig. 2).

Sensor ParticularsWhen using the HydroNit sensor,

the furnace atmosphere is measureddirectly. This is in contrast to othersensors where the furnace atmos-phere is pumped out of the furnacethrough hoses or pipes beforereaching the sensor. The chemicaland thermochemical reactions thatoccur in the gas as it travels from thefurnace to the sensor can affect themeasured value and can potentiallyinvalidate the calculated Kn.

The sensor consists of a protectiveshield that is inserted into the retort(Fig. 3). This shield contains a meas-uring tube made of a material per-meable to hydrogen only. Therefore,the result cannot be affected by cross-sensitivities from any other compo-nents of the furnace atmosphere.

The sensor is evacuated at the be-ginning of a heat treating process.During nitriding, the ammonia dis-sociation process causes a differencein hydrogen concentration and par-tial pressure between the furnace at-mosphere and the interior of themeasuring tube. This difference al-lows the hydrogen to diffusethrough the wall and into the meas-uring tube. Diffusion continues untilthe concentrations and pressuresequalize. That is, the pressure meas-ured by the sensor is equal to the

52 HEAT TREATING PROGRESS • MARCH/APRIL 2008

Fig. 2 — HydroNit sensor.

Furnace wall

Head

Protection tube

Measurement tubeQuartz tube Pressure sensor

Furnace atmosphere

NH3/H2/N2/CO/CO2

Measurement tube

pa(H2) = pi(H2)pa(H2) pi(H2) H2 Pressure balance

Fig. 3 — The HydroNit sensor consists of a protective shield that is inserted into the retort.

furnace atmosphere hydrogen par-tial pressure.

Kn is calculated after the measure-ments are completed, and it can beused to control the heat treatmentprocess. The nitriding potential canbe programmed in the hold segmentof a nitriding process recipe. Whenthe hold segment is active, the pro-grammer continuously compares thedesired Kn to the Kn calculated fromthe data measured from the furnaceatmosphere.

If the calculated Kn is below the de-sired value, nitriding will not occuras expected. In this case, the hy-drogen flow rate is reduced until thecalculated Kn matches the desiredvalue. If reducing the hydrogen flowrate does not increase the calculatedpotential to match the desired poten-tial, the ammonia supply flow ratecan be increased.

If the calculated potential ishigher than desired, the ammoniaflow rate can be reduced until thedesired value matches the actualvalue. However, if the ammonia isreduced to the minimum flow raterequired to safely run the processand the nitriding potential is stillhigher than desired, the hydrogenflow rate can be increased. Aschematic circuit for this type of con-trol system is shown in Fig. 4.

Benefits of Using the SensorThe use of the nitriding potential

and a process management systemto regulate the process significantlyimproves nitriding and nitrocarbur-izing process consistency and re-peatability. The calculated Kn, atmos-phere flow rates, furnace pressure,and temperature can be recorded. Inthe event that there is an issue withnitriding a load, the process data forthat specific load can be comparedto process data for previous loads toquickly identify any problems andto take in-process corrective actionto prevent any adverse impact onheat treating results.

This is illustrated in examplesshowing laboratory results of loadsrun in nitriding and nitrocarburizingatmospheres with and without pre-oxidation.

Example No. 1: The following re-sults are from a nitrided batch of ringgears (Fig. 5) made of 31CrMoV9V(equivalent to 1.8519). The treatmenttime was 70 hours at 960°F (515°C)with the Kn controlled at approxi-mately 4.

Desired Actual

Surface hardness, HV10 720-820 758

Case depth, mm 0.4-0.75 0.46

Max white layer thickness, µm 20 17

Figure 6 shows the white layer on thepart surface.

Example No. 2: The following re-sults are from nitrocarburizing dif-ferential pins (Fig. 7) made of31CrMoV9V. The treatment time was18 hours at 1000°F (540°C) with theKn controlled at approximately 3.

Desired Actual

Surface hardness, HV1 750-950 823

White layer thickness, µm 14-22 16.2

HEAT TREATING PROGRESS • MARCH/APRIL 2008 53

Fig. 4 — Schematic of nitriding potential control with flow control

Furnacevol% H2

Kn T

HydroNit pH2 Nitr-o-Prof

sensor Kn set-point

H2

NH3

Fig. 5 — Nitrided 31CrMoV9V ring gears.

Fig. 6 — Microstructure of nitrided parts (inFig. 5) showing white layer.

Figure 8 shows the white layer on thepart surface.

Example No. 3: The following re-sults are from nitrocarburizing in-ternal geared wheels (Fig. 9) made of42CrMo4 (equivalent to 1.7225). Thetreatment time was 5 hours at 1060°F(570°C) with the Kn controlled at ap-proximately 2.5.

Desired Actual

Surface hardness, HV10 550-650 630

Case depth, mm 0.4-0.55 0.50

White layer thickness, µm 8-35 14.3

Figure 10 shows the white layer onthe part surface.

Example No. 4: The following re-sults are from nitriding hubs (Figs.11 and 12) made of 16 MnCr5 (equiv-alent to 1.7131). The treatment timewas 14 hours at 970°F (520°C) with

the Kn controlled at approximately3.5.

Desired Actual

Surface hardness,HV1 550-750 719

Case depth, mm 0.30-0.60 0.39

The development of process sensors,such as the HydroNit, and computercontrol systems has proven to be in-valuable for the nitriding process. Ni-triding and nitrocarburizing processconsistency and repeatability haveimproved considerably. HTP

For more information: Patrick Weymer,Ipsen Inc., 984 Ipsen Rd., Cherry Valley,IL 61016; tel: 815-332-2539; e-mail:Patrick.weymer@ipsenusa.com; Web site:www.ipsenusa.com.

54 HEAT TREATING PROGRESS • MARCH/APRIL 2008

Fig. 9 — Nitrocarburized 42CrMo4 internalgeared wheels.

Fig. 10 — Microstructure of nitocarburizedparts (in Fig. 9) showing white layer.

Fig. 8 — Microstructure of nitocarburized parts(in Fig. 7) showing white layer.

Fig. 7 — Nitrocarburized 31CrMoV9V differential pins.

Fig. 11 — Nitrided 16MnCr5 hubs. Fig. 12 — Nitrided 16MnCr5 hub.