posted
On Friday, November 17, 2023
in
Blog
The connection between mechanical engineering and materials science is essential for designing and manufacturing reliable machines and various machining and metal-forming tools [1]. One of the few methods, very effective in shaping surface properties of the mechanical components, is nitriding. Nitriding applications are steadily growing [2-11]. This is the effect of the importance and appreciation of this thermochemical surface engineering technique allowing for processing mechanically finished components and tools at comparatively low temperatures without a negative effect on their bulk/core properties.
Precise control of the gas nitriding process has been improved dramatically in the last 25 years. The process, which was purely controlled in the past by measuring the dissociation rate of ammonia in the furnace chamber, is now executed with full control of the nitriding potential, availability of nitrogen and when dosed with carbon or oxygen, with the respected potentials of those elements. The same applies to plasma/ion nitriding, which has been used in its best applications, including stainless steel parts, sintered metal products and parts requiring partial treatment [1]. This process can be continually monitored by visual methods since the glow discharge is very intense, see figure 1.
Figure 1: Plasma nitriding of 140” gear with the internal and outside teeth
Picture taken at Advanced Heat Treat Corp., Monroe, Michigan.
In both methods, very precise control of the process allows for the desired formation of the structure and properties of nitrided layers and as a result, properties of the steel parts [1-5].
Residual Stress & Surface Hardness
Nitriding and nitrocarburizing are widely applied surface treatments for improvement of wear, corrosion and fatigue properties of ferritic as well as austenitic steels [6].
Mechanical effects, such as increasing surface hardness of the steel as well as formation of the compressive residual stresses near the surface, are the main results of nitriding [7]. Residual stress states are—intentional or not— characteristic consequences of individual manufacturing steps in the course of a process chain. The basic principles of residual stress formation depending on the process parameters applied are well known today and fast and reliable methods for stress analysis are available. This is a key factor for the successful manufacturing of advanced materials and structures with short production cycles and reduced production costs. Basic and promising approaches exist to introduce residual stress effects in design rules of highly stressed components to increase the safety and reliability of the parts. As a consequence, research about causes and consequences of residual stresses as well as residual stress engineering—the appropriate application and extension of existing knowledge about residual stresses—are well-established fields both in basic materials research as well as in industrial applications.
As an example, a schematic representation of the stresses present in a nitrided shaft is shown in figure 2.
Figure 2: Nitrided steel shaft subjected to bending stress
Note that the resultant stress has its maximum below the surface. Drawing adopted from [4].
Nitrided steel shafts subjected to bending and rotation have a resulting maximum stress below the surface and it is minimized compared to the stress in untreated parts. Therefore, their fatigue life will be significantly increased since the fatigue and crack behavior depend both on the strength of the surface layer and the core and on the residual stresses within the case and the core. The characterization of the mechanical behavior of surface compounds needs to be done depending on the load case, including the determination of local and global properties of the materials compound. The large microstructure and property gradients in nitrided cases demand testing methods with a high spatial resolution for determining the local properties. Therefore, the indentation methods with their load-depending resolution belong to the fundamental test methods for characterizing the mechanical properties of nitrided layers. Figure 3 and 4 shows the mechanical properties of a very deep nitrided layer formed in the steel used for making large gears.
Figure 3: Wavelength Dispersion Spectroscopy (WDS) Nitrogen & Carbon Content Profile
WDS nitrogen and carbon content profile in European E33CrMoV12-9 after nitriding. Adopted from [10].
As seen in figure 3, nitrogen penetrated the steel to about 0.8 mm. At the same time carbon distribution changed by decarburizing near the surface as well as the “nitrogen pushing effect” toward the core. All of this resulted in a significant increase of hardness to the depth up to about 0.8 mm and formation of the residual compressive stress to about -400 MPa maximum, see figure 4.
Figure 4: Hardness (left) and Residual Stress (right) Steel Profiles after Nitriding
Profiles in E33CrMoV12-9 steel after nitriding. Adopted from [10].
For the service behavior of nitrided components, different areas are of importance, depending on the depth effect of the load [10]. The effect of tribological stresses, for example, is preferentially confined on the direct surface layer. In contrast to this, the fatigue and crack behavior depend both on the strength of the surface layer and the core and on the residual stresses within the case and the core. Thereby, the transitions between the single load cases are fluent. The large microstructure and property gradients in nitrided cases demand testing methods with a high spatial resolution for determining the local properties. Therefore, the indentation methods with their load-depending resolution belong to the fundamental test methods for characterizing the mechanical properties of nitrided layers. The conventional hardness measurement, which can often be correlated to other mechanical properties, is still successfully applied for evaluating the surface hardness and the hardness distribution at cross-sectional and angle polished samples.
The wear resistance and tribological behaviors of nitrided components are of significant importance for designing engineers. Nitriding increases those properties very much as demonstrated in Figure 5.
Figure 5: Linear Wear Depth vs Time of Friction in Nitralloy 135M
Linear wear depth vs. time of friction at different loads of nitrided layer produced in Nitralloy 135M. Total case depth 0.10mm (3.9 mils), compound layer was 0.009mm, and surface hardness was 1300 HV).5. Linear wear at 50-200 MPa (7 to 29 ksi) was 4.3 to 7.6 µm, and wear intensity was 0.03 to 0.027 µm/min. Adopted from [11] .
Nitriding Tips for Abrasion, Adhesion, Corrosion & Fatigue
Nitriding of the objects shown in figures 1 and 6 increase their tribological as well as fatigue properties such as bending and rolling contact fatigue (RCF).
General recommendations for the designing engineers considering nitriding of their products made of steel could be summarized as follows [12]:
1. Parts subjected to abrasive and adhesive wear should have nitrided layer with the compound zone of the ɛ (N, C)- type thicker than 10 µm (0.0004”) with a possible oxide on the top, high surface hardness and hard diffusion zone.
2. Parts subjected to corrosion should have nitrided layer with the compound zone of the ɛ(N,C)-type and thickness 15-20 µm (0.0004-0.0008”). The thickness of the diffusion zone is less important.
3. Parts subjected to contact fatigue should have a layer with minimum or zero compound zone, high surface hardness >600 HV0.1, diffusion layer >0.35 mm (0.014”), good toughness and proper distribution of internal stresses.
4. Parts subjected to bending and rotational fatigue should have a layer with similar characteristics as above in p.3 except that the presence of a compound zone is more tolerated as long as it does not have any cracks.
Do you need a heat treat solution for abrasion, adhesion, corrosion or fatigue? Contact AHT.
FILL OUT OUR FORM
References for "Nitriding Steel: Abrasion, Adhesion, Fatigue & Surface Hardness"
1. Advanced Materials & Processes, Discussion of five Experts, “Bridging Mechanical Engineering and Materials Science”, October 2023, Vol. 181, No 7, pp. 12-17.
2. E. Rolinski, A. Springer and M. Woods, “PRACTICAL ASPECTS OF PLASMA NITRIDING KINETICS FOR 17-4 PH STAINLESS STEEL”, Advanced Materials and Processes, 2022, July/August, 56-59.
3. T. Bell “Gaseous and plasma nitrocarburizing”, in ASM Handbook,
4, Heat Treating, Materials Park, OH, AMS International,1991, pp. 425–436. 4. T. Bell and N.I. Koh, Journal of Heat Treating, American Ed. American Society for Metals, Vol. 2 No 3, June 1982, pp. 232-237.
5. E. J. Mittemeijer, “Fundamentals of Nitriding and Nitrocarburizing”, ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes J. Dossett and G.E. Totten, editors, 2013, ASM International, pp. 619-646.
6. Thermochemical Surface Engineering of Steels, Ed. E. J. Mittemeijer and M. A. J. Somers, Pub. Woodhead Publishing, 2014.
7. E. Rolinski,” Plasma Assisted Nitriding and Nitrocarburizing of Steel and other Ferrous Alloys”, Chapter 11 in Thermochemical Surface Engineering of Steels, Ed. E. J. Mittemeijer and M. A. J. Somers, Pub. Woodhead Publishing, 2014, pp. 413-449.
8. E. Rolinski, G. Sharp, “Controlling Plasma Nitriding”, ASTM International, Materials Performance and Characterization, Vol. 6, No 4, 2017, pp.698-716, https://doi.org/10.1520/ MPC20160051. ISSN 2370-1365.
9. M. A. J. Somers, E. J. Mittemeijer, “Development and Relaxation of Stress in Surface Layers; Composition and Residual Stress Profiles in ɣ’ Fe4N1-x Layers on ɑ-Fe Substrates”, Metallurgical Transactions A, Vol. 21A Jan. 1990, pp. 189-203.
10. S. Thibalt, C. Sidoroff, S. Jegou, L. Barallier, G. Michel, “A Simple Model for Hardness and Residual Stress Profiles Prediction for Low-Alloy Nitrided Steel, Based on Nitriding-Induced Tempering Effects”, HTM J. Heat Treatm. Mat., 73 (2018) 5; pp. 235-245
11. J. Senatorski, J. Tacikowski, E. Rolinski and S. Lampman, “Tribology of Nitrided and Nitrocarburized Steels”, ASM Handbook Vol 18, Friction, Lubrication and Wear Technology, ed. G. Totten ASM International, 2017, pp.638-652.
12. H. J.; Spies, A. Dalke, “Case Structure and Properties of Nitrided Steels”. In Comprehensive Materials Processing; Krauss, G., Ed.; Elsevier Ltd., 2014; Vol. 12, pp 439–488.
- edward rolinski
- ion nitriding
- nitriding
- plasma nitriding
- stainless steel