13Hotline 143

The need to ensure reliable nitriding andnitrocarburising treatments becomes incre-asingly important every year as equipmentages and tighter metallurgical require-ments and full compliance to specificationare demanded. In many instances, it’spossible to push existing equipment backinto useful life with the application of newcontrol and sensor technology. Using asolution-oriented approach, a selection ofappropriate technologies can be identifiedbased on process and specification require-ments. This article examines controlequipment and sensor technology up-grades that address the challenge.

STANDARDS

AMS 2759/6 (Nitriding)Without proper control standards, themechanical properties of parts may becompletely differentfrom one charge toanother. The industry’s firstattempt at nitridingspecification was 25years ago with therelease of AMS2759/6, based oncontrol of ammoniadissociation via per-iodic burette meas-urement during theprocess (Fig.1), thenecessary adjust-ments being donemanually.AMS 2759/6 provisions include the following:• Atmosphere control: Equipment shall be

available to measure and maintain the dissociation of the process atmosphere in the retort within ±5% of the selected percent dissociation throughout the nitriding cycle.

• Ammonia dissociation: The ammonia gas dissociation shall be 15 to 35 percent for (single-stage) Class 2 and for the first stage of (two-stage) Class 1 (approximately 20 percent of total nitriding time). Ammonia dissociation for the second stage of Class 1 shall be 65 to 88 percent.

AMS 2759/10 (Nitriding)A refinement to the earlier specification,

AMS 2759/10, issued in 1999, allows theuse of diluted atmospheres (i.e. nitrogen +ammonia). It also calls for “AutomatedGaseous Nitriding Controlled by NitridingPotential”Nitriding potential or KN determines thenitrogen concentration at a given temper-ature. This controlling parameter in nitridingis defined as: KN = pNH3/pH2

3/2

AMS 2759/10 provides the framework forprocess control using nitriding potential KN

as a reference point. The Lehrer diagram(Fig.2) illustrates the iron-nitrogen equil-ibrium at various temperatures andhydrogen/ammonia levels as defined byKN. Note the relationship of KN to phase.

Solution-oriented approach tonitriding and nitrocarburising

Pat Torok, United Process Controls, USA

Fig. 1: Burette forammonia

dissociationmeasurement.

AMS 2759/10 classifies the nitridingprocesses described as follows:• Class 0: No white layer permitted. • Class 1: 0.0005inch (0.013mm) maximum

white layer thickness permitted.

Table 1. Recommended ranges of nitriding potential values. (From AMS 2759/10) Alloy

Class 0

Class 1

Class 2

Stage 1

Stage 2

Stage 1

Stage 2

Stage 1

Stage 2

Nitralloy 135M

4-12

0.3-0.8

4-12

0.6-1.8

6-15

1.2-2.6

Nitralloy EZ

4-12

0.3-0.8

4-12

0.6-1.8

6-15

1.2-2.6

Nitralloy N

4-12

0.3-0.8

4-12

0.6-1.8

6-15

1.2-2.6

4140, 4340

4-12

0.25-0.7

4-15

0.6-2.6

4-15

1.2-4.5

D6, D6AC

4-12

0.25-0.7

4-15

0.6-2.6

4-15

1.2-4.5

H11

5-15

0.3-0.8

5-15

0.4-0.9

5-15

2.2-5.5

Stainless

5-15

0.2-0.7

5-15

0.4-0.9

Not recommended

Carbon steels

N/A

N/A

5-12

0.8-2.6

1.2-4.0

N/A

Fig.2: Fe-N equilibrium diagram KN -T (Lehrer).

This article is basedon Pat Torok’spresentation atthe CHTA-co-sponsored SurfaceEngineering & HeatTreatment IndustryConference inStratford-upon- AvonUK on 16 October2015.

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Issue No. 143March 2016March 2016Issue No. 143

14 Hotline 143

PROCESSING

Table 3. Ferritic nitrocarburising process parameters ensuring the formation of an

epsilon-type compound layer with two ranges of porosity level. (From AMS 2759/12A)

Material

Process temper-

ature

Process time

(hours)

Class 1

Class 2

Porosity not exceeding 15% of thickness of

compound layer

Porosity above 10% but not exceeding 50% of

thickness of compound layer KN

KC

KN

KC

ºF

ºC

min max

min max

min max

min max

Group 1*

1040 1075

560 579

3-6 2-5

2.13 1.50

2.41 1.60

0.57 1.10

0.69 1.22

2.48 1.68

2.68 1.78

0.49 0.86

0.54 0.94

Group 2**

980

527

6-30

4.51

5.55

0.16

0.24

6.03

7.10

0.09

0.13 Group 3***

1060

571

3-10

1.82

2.10

0.76

0.99

2.22

2.64

0.48

0.68 *Group 1: HSLA, carbon steels **Group 2: 4140, 4340, Nitralloy 135M ***Group 3: Cast iron

Table 2. Limits for the thickness of the compound layer for various materials.

(From AMS 2759/12A)

Material

Thickness,

inch

Thickness,

mm

min

max

min

max

Carbon and

HSLA steel

0.0002

0.0010

0.0051

0.025

Low-alloy steel

0.0002

0.0010

0.0051

0.025

Tool steel

0.0001

0.0006

0.0025

0.015

Cast iron

0.0002

0.0010

0.0051

0.025

Fig.3: The elementsof fully-automatedprocess control of

nitriding/nitrocarburising/post-oxidation,

showing all the gassupply options.

• Class 2: 0.001inch (0.025mm) maximum white layer thickness permitted.

• If no class is specified, Class 2 applies. Class 2 is not recommended for corrosion-resistant steels.

AMS2759/10 goes on to provide recom-mendations for various material types andnitriding classifications (Table 1). Thespecification also dictates permissibledeviation from KN set point for both stagesof the process.

AMS 2759/12 (Nitrocarburising)Filling the gap for ferritic nitrocarburisingcame AMS 2759/12, “Gaseous Nitro-carburizing, Automatically Controlled byNitriding and Carburizing Potentials”.The aim of nitrocarburising is to build acompound layer on the surface of theworkpiece, typically consisting of epsiloncarbonitrides. In addition to the ammoniathat is used in all nitriding processes, acarbon-bearing gas is also added. Thecarburising effect of the atmosphere,caused by the addition of CO or CO₂ orother gas containing these constituents,can be calculated.The carburising potential KC can be

defined based on one or another of tworeactions:• the Boudoir reaction: 2CO = CO₂ + C, for

which:

p²COKCB = -------

pCO₂

• the water-gas shift reaction: CO + H₂O = CO₂ + H₂ ,for which:

pCO x pH₂

KCW = --------------pH₂O

Note: KCB and KCW will not give the samenumbers!ASM 2759/12 establishes limits for thethickness of the compound layer forvarious materials (Table 2); it also indic-ates process values for KN and KC toensure an epsilon-type compound layerwith two ranges of porosity level in “Class1” and “Class 2” ferritic nitrocarburising(Table 3).

CONTROLLING THE POTENTIALSFig.3 illustrates the typical configuration offully-automated process control and all the

gas supply options. It comprises acontroller with the capability of computingKN or KC for nitriding and nitrocarburisingprocesses, electronic flow control ofvarious gases, a hydrogen analyser, anoxygen probe for KC measurement and anoptional ammonia dissociator.

Fig.4: (a) Ammonia flow versus KN ; (b)Ammonia / dissociated ammonia flow versus KN .

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15Hotline 143

PROCESSING

Fig.5: Effect of nitrogen dilution ontransfer coefficient, k3. (570°C, KN = 1) Fig.6: Defined nitriding and carburising potentials.

NitridingLet’s take the simplest case: one processgas: ammonia. The required controlelements are electronic flow control ofammonia, hydrogen analyser for determ-ining KN, and a controller that has thecapability to compute and control KN.Nitriding potential is controlled bymodulating ammonia flow (Fig.4a). When low KN set points are desired,furnace flow and pressure may too low.This problem can be solved by theaddition of dissociated ammonia (Fig.4b).The other advantage of this controlscheme is that it is possible to obtain verylow KN values which are necessary for low, orno, white layer. If dissociated ammoniais not available, dilution with nitrogen willdefinitely solve the furnace pressureproblem. This approach introduces twoother problems: the transfer coefficient ofnitrogen is affected and there is a problemwith the KN calculation. Atmospheredilution of 3:1 lowers the nitrogen transfercoefficient by a factor of 5 at constant KN

(Fig.5).Diluting ammonia with nitrogen will reduceKN down to a certain minimum, determinedby the availability of ammonia. When thereare not enough ammonia molecules in theatmosphere, KN will go up.

NitrocarburisingCarbon and nitriding potentials are inter-related. An adjustment of one potentialrequires the adjustment of the other. PerAMS 2759/12, selected pairs of KN and KC

are always aiming to achieve epsiloncarbonitrides with defined percentages ofnitrogen and carbon in the compoundlayer (Fig.6).Nitrocarburising is typically aiming for acompound layer of Fe2-3N or Fe4N ironnitrides of high hardness, increasingcorrosion resistance and lowering thecoefficient of friction.By defining the weight percentages ofcarbon and nitrogen in the compoundlayer, porosity and corrosion resistancecan be controlled. The micrographs in

Fig.7 illustrate results from different wt%nitrogen and wt% carbon. Pitting potentialalso increases with total wt% of carbonand nitrogen greater than 8.5% in thecompound layer (Fig.8).CO2 may be used as the carbon-bearinggas in the nitrocarburising process. How-ever, using CO2 as the only carburisinggas forces a reversed behaviour of thecontrol loop, as increasing CO2 results in alower KC. Remember, too much CO2 canbe decarburising.Endothermic gas as the carbon-bearingaddition enables high KC. A lower KC canbe achieved by adding CO2 or air.However, the disadvantage in this case is

that the high hydrogen content in endohinders high nitriding potentials.Fig.9 shows carbon potential for thevarious gases. Note that, with endo, weend up with very high carbon potentials.Therefore it is advisable to have the “gas”and the “brake”. This can be done with acombination of endo and CO2 or air.

Fig.8: Pitting potential with increasedtotal nitrogen and carbon concentrations. Fig.9: Various carburising gas choices.

Fig.7: Compound layer porosity with various carbon and nitrogen contents.

Fig.10: Control behaviour in a realnitrocarburising process (570°C, 4hours).The controller is able to react to varyingprocess conditions so that both potentials

(KN = 1; KCB = 0.3) are kept constant.

16 Hotline 143

PROCESSING

One of the challenges for control is thespeed of reaction of the control system.Nitrocarburising cycles are short com-pared with nitriding durations. Atmospherepotentials must be established at the verybeginning of the cycle to achieve thedesired compound layer composition.Potentials must be controlled to achievethe desired epsilon phase. This includesheat up and cool down (Fig.10).Relative transfer times are affected byprocess pressure. The nitrogen uptakecan be expressed as a function of thehydrogen partial pressure. There is a six-fold increase in transfer time from a H2

partial pressure of 0.09bar to 0.33bar(Table 4).

Post oxidationPost oxidation is very effective way topromote corrosion resistance. Controlgases are H2O or N2O vs. NH3 or H2.Control and measurement can beachieved with the use of an oxygen probe.A 13µm epsilon compound layer with anadditional magnetite oxide layer providesfour times better corrosion resistance than40µm chrome plating. Using the signal ofthe oxygen probe enables a controlledpost-oxidation process aiming for bestcorrosion resistance.

FULLY-AUTOMATED PROCESSCONTROLAtmosphere must be analysed contin-uously. The system must be capable ofcalculating KN, KC, and KO and of adjustinggas flows to maintain the process setpoints. Elements of the system are: a pro-grammable controller that can calculatepotentials and interface with sensors,electronic flowmeters and temperaturecontrol devices. The controller should alsobe able to store a large number of processrecipes, have logging capability, for dataanalysis, and a webserver for easy

maintenance and reporting (Fig. 11a).Electronic flowmeters should be accurate,with precise calibration curves such aspolynomial fits, and feature modern com-munications that will eliminate analogsignal error such as Ethernet or Modbus/

TCP. They should also be easy to main-tain and feature a built-in webserver(Fig.11b).Hydrogen measurement demands built-inprecise temperature control of themeasuring cell and accurate continuously-variable flow control. It is also imperativethat this device has extremely low drift andpolynomial calibration (Fig.11c).For nitrocarburising and post oxidation, anoxygen sensor that is operational atprocess temperatures is a necessity. Theelectrode impedance must remain below50kohms in order to produce a reliablesignal. The sensor design must alsoaccommodate high dewpoints during partsof the cycle without resulting in sensordamage (Fig.11d).

SUMMARYSensor and control technology exists toensure process control that is compliantwith specifications such as AMS 2750,AMS 2759 and CQI-9. This technologyalso features sensor and control com-ponents that are reliable enough to take fullrecipe control of the process.A technology upgrade to an existingfurnace will add years of service andperformance to equipment that might havebeen underutilised.

BIBLIOGRAPHY• Winter K. M. (2009). Phase-controlled gaseous nitriding and nitrocarburizing. Process Electronic.

• ASM (2003). Practical Nitriding and Ferritic Nitrocarburizing. American Society for Materials.

• Winter K. M. (2011). Nitrocarburizing with independently-controlled nitriding and carb-urizing potentials. Process Electronic.

FURTHER INFORMATIONPat Torok is based at United ProcessControls, 8904 Beckett Rd, West Chester,Ohio 45069, USA (Pat.Torok@group-upc.com).

Table 4. Effect of pressure on nitrogen transfer time

Total pressure / nitrogen dilution

Hydrogen

partial pressure

Relative nitrogen transfer speed

1bar / 0%

0.33bar

0.30

0.8bar / 20%

0.28bar

0.23

0.5bar / 50%

0.19bar

0.13

0.2bar /80%

0.09bar

0.04

w

Ta

elbTa 4. noerusserpfotcefEfemiteransfrtogen rtni

altToe essurpr

ogen rtnioiutldi n

0%/1bar

20%/8bar0.

50%/5bar0.

80%/2bar0.

emiteransfrtogen rtni

/e ogen

n

negordHyalitpar

eessurpr

evitalReogen rtni

rfesnatrspeed

0% 33bar0. 300.

20% 28bar0. 230.

50% 19bar0. 130.

80% 09bar0. 040.

Fig.11: Instrumentation in a fully-automatedprocess control system.

Surface Engineeringand Heat TreatmentIndustry Conference►► 13 October 2017 ◄◄Stratford-upon-Avon, UK

Co-sponsored by…

CONTRACT HEAT TREATMENT ASSOCIATIONWOLFSON HEAT

TREATMENT CENTRE

Repeat scheduled!Following the success of last October’s event, the Surface

Engineering and Heat Treatment Industry Conference, Exhibitionand Dinner will be staged again in October 2017.

Information for delegates, sponsors and table-top exhibitorswill be published in a forthcoming edition of Hotline.

OFFERS OF PRESENTATIONS WELCOMEMeantime, CHTA’s Alan J Hick will again be compiling the

programme for the heat treatment sessions. He’ll be grateful foroffers of presentations, on advances in industrial heat treatment

processing, to mail@chta.co.uk.

(a) Programmablecontroller

(c) H2Smart H2 analyser (b) Electronicflow control

(d) UPCNitrocarb O2 sensor

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