The Success of a Nitriding Process Depends on the Ability to Meet Metallurgical Requirements Involving Microstructures, Surface

Home , Nitriding

QUALITY COUNTS: GAS NITRIDING The success of a nitriding process depends on the ability to meet metallurgical requirements involving microstructures, surface hardness, case hardness, and, in some cases, the part’s appearance. By Jim Oakes

In general, nitriding of parts involves a thermal process that provides a tough, corrosion-resistant, and wear-resistant surface with less distortion compared to other case hardening processes due to processing temperature and no need for quenching. There are different methods of nitriding, including gas nitriding, plasma/ion nitriding, and salt-bath nitriding. This article focuses on gas nitriding. When nitriding gears, it is common to see requirements that specify no white layer for the finished part. The white layer, also known as the compound layer, is a hard, brittle layer that is formed during nitriding but can be reduced or virtually removed through process control. Although the white layer can be machined off, the typical goal would be to reduce or even eliminate the post-nitride machining. This can be accomplished due to the fact that the part’s dimensional integrity remains uncompro- mised from distortion through lower processing temperatures and KN is mathematically defined in Equation 1: lack of a quench. 3/2 Equation 1 For gas nitriding, the process variables are time, temperature, and KN = pNH3/(pH2) atmosphere. Processing temperature for nitriding will most often be between 975°F and 1050°F (524°C and 566°C), but it can be as low as where p denotes partial pressure. 650°F (343°C) for certain applications. The parts will be exposed to a non-oxygen-bearing, nitrogen-rich atmosphere typically produced Many specifications require tight tolerances on the amount of white by a mixture of nitrogen, ammonia, and dissociated ammonia. The layer on the surface, which requires a control system to monitor the furnace atmosphere will be continuously replenished with an atmo- furnace atmosphere and control the potential of nitrogen that is available sphere that provides a potential of nitrogen to diffuse into the steel to react at the surface of the part. To perform continuous closed-loop and form nitrides. control, there must be a method of measurement for the atmosphere. In order to meet specifications for nitriding, a common control In the past, the measurement was discontinuous using a water burette variable used to measure the amount of nitriding is KN (nitriding that would provide furnace operators with a method of measuring the potential). KN is a derived measurement of an atmosphere’s potential percentage of residual ammonia in the atmosphere. If the residual ammo- to allow for the diffusion of nitrogen into a material — specifically, nia is available, the percentage of dissociated ammonia (% DA) can be iron, in this case. determined and then analyzed to adjust the flow rates of the process gas.

14 | Thermal Processing for Gear Solutions To control the gases introduced to the with controlling the white layer thickness. ABOUT THE AUTHOR: Jim Oakes is vice furnace, today’s automated controls use feed- The atmosphere can be controlled to elimi- president of business development for Super back from the atmosphere being measured. nate the white layer or at least reduce it to Systems Inc. (SSi), where he oversees marketing For gas nitriding, process variables used in less than 0.007 mm (0.0003") for minimal and growth in multiple business channels and this thermochemical treatment are repre- post-nitride machining/grinding. Although helps develop product innovation strategies in sented by nitrogen, dissociated ammonia beneficial for some applications, the white conjunction with customer feedback. He has extensive experience working in the heat treating (bottled or from a dissociated ammonia gen- layer may not be desired in the process based and software/IT industries. For more information, erator), and ammonia. Continuous measure- on manufacturing steps and performance email him at [email protected] or go to ment of the exhaust gas using a hydrogen requirements for the finished goods. www.supersystems.com. analyzer provides a method of closed-loop control by varying the process gases to meet a desired control variable setpoint. In this case, atmosphere control variables are KN/DA and gas flows, which will facili- tate delivery of nitrogen to the processed part. The measurement of hydrogen in the exhaust provides enough data to calculate DA or KN. When ammonia breaks down to one-part nitrogen and three-parts hydrogen, the hydrogen can be measured to determine the percentage of uncracked ammonia in the atmosphere. For example, if we measure 30-percent hydrogen in the exhausted atmosphere, that would leave us with 10-percent nitrogen, meaning that 40-percent of the atmosphere is dissociated ammonia. Our residual 60-percent would be represented by ammonia. The assumption would be that only DA gas and ammonia were used as supply gases. If the atmosphere is using N2 for blending, the volume of gas flowing into the retort of the furnace will be used in our calculation. By understanding the amount of ammonia dissociated in the exhaust gas, SOLUTIONS the nitrogen activity on the surface can be SOLUTIONS

deduced. The greater the flow of ammonia, SOLUTIONS the greater the amount of ammonia measured SOLUTIONS Ajax TOCCO is YOUR Complete Source

SOLUTIONS Ajax TOCCO is YOUR Complete Source in the exhaust and the more ammonia to SOLUTIONS for forInduction Induction Heat Heat Treating Treating Systems, Systems, which the parts are exposed. The higher the Ajax TOCCOPower is Supplies YOUR Complete & Installation! Source amount of ammonia is present, the lower the Power Supplies & Installation! AjaxforAjax Induction TOCCO TOCCO is HeatYOUR is YOUR Treating Complete Complete Systems, Source Source amount of dissociation (DA), and the greater forPower Induction Supplies Heat & TreatingInstallation! Systems, the activity of nitrogen on the surface. Too zfor Scanners PowerInduction z Supplies Single Heat Shot Treating& z Installation! Lift & Rotate Systems, z Tooth by Tooth z Part Development z Scanners Powerz Single Supplies Shot z Lift & &Installation! Rotate z Tooth by Tooth z Part Development much ammonia could lead to a significant Powerz System Supplies Reliability & Services Installation! z Single-source Installation Packages compound layer, nitride networking, and a z z Systemz Reliability Servicesz z Single-sourcez Installationz Packages z Scannersz Commercial z Single Heat Shot Treat z Lift z &Parts Rotate & Service z Tooth z byQuality Tooth Monitoring z Part Development Systems brittle surface. z zCommercial z Scanners z Heat Single Treat Shot z z Parts Lift & &z Rotate Service z Tooth z Quality by Tooth Monitoring z Part Development Systems z z Systemz z Inductor Reliability Design, Servicesz Repair z & Single-source Rebuildz z Preventive Installation zMaintenance Packages Two-stage nitriding processes are used Scanners zSystem zInductor System Single Reliability ReliabilityDesign, Shot Services Repair Lift Services & &Rotate Rebuild Single-sourcez Single-source Tooth z Preventive by InstallationTooth Installation Maintenance Part Packages Packages Development ENGINEERED z Commercial Heat Treat z Parts & Service z Quality Monitoring Systems to create proper surface and case condi- z Commercialz System Reliability Heat Treat Services z Parts &z ServiceSingle-source z Quality Installation Monitoring Packages Systems ENGINEERED z24/7 Commercial Customer Heat Service Treat z Parts & Service z Quality Monitoring Systems tions. Similar to a carburizing boost and 24/7z Customer Inductor Design, Service Repair & Rebuild z Preventive Maintenance z Commercialz800-547-1527 Inductorz Inductor Heat Design, Design,Treat Repair z RepairParts & & Rebuild& Service Rebuild z z zPreventive QualityPreventive Monitoring Maintenance Maintenance Systems diffuse method, two-stage nitriding utilizes ENGINEERED

ENGINEERED 800-547-1527z z ENGINEERED 24/7 Customer Inductor Design, Service Repair & Rebuild Preventive Maintenance a nitrogen-rich atmosphere for the buildup 24/724/7 Customer Customer Service Service 1745 Overland Avenue ENGINEERED Induction OEM for: of nitrogen concentration in the steel (a.k.a. 24/7800-547-1527 Customer Service Warren,1745 Overland Ohio USA Avenue 44483 the first stage) and a reduced nitrogen Induction OEM for:800-547-1527 Warren, P:Ohio 330-372-8511 USA 44483 800-547-1527 1745 Overland Avenue atmosphere to allow for diffusion of the 1745 OverlandP:F: 330-372-8511330-372-8608 Avenue Induction OEM for: Warren,1745 OverlandOhio USA Avenue 44483 nitrogen into the steel (a.k.a. the second InductionInduction OEM for: OEM for: Warren, Warren, F: OhioOhio 330-372-8608 USA 44483 www.AjaxTocco.com 1745 OverlandP: 330-372-8511 Avenue stage). This method is most commonly used Induction OEM for: P:P: 330-372-8511330-372-8511 www.AjaxTocco.com Warren, F:Ohio 330-372-8608 USA 44483 thermalprocessing.comP:F:F: 330-372-8511330-372-8608330-372-8608 | 15 www.AjaxTocco.comwww.AjaxTocco.com F: 330-372-8608 www.AjaxTocco.com