Parts undergoing ion nitriding surface treatment in process chamber

Ion (also called plasma) nitriding and ion nitrocarburizing can be used in applications where good wear, antigalling properties and erosion resistance of engineering components and products are required. Ion nitriding improves fatigue and frictional properties of components, and also, in many situations, enhances corrosion resistance. This is a low temperature (800-1100 F, or 430-595 C) process; therefore, there is very little or no distortion even if a very significant residual compressive stress is induced in the surface layer of the treated products.

Fig 1. Photomicrograph of the hub synchronizer surface layer after ion nitrocarburizing at 1040 F (560 C). Hub material is 0.3-0.6 %C, 1.5-3.9 %Cu, bal Fe, sintered low-density (7.1 g/cm3) PM. Note a white compound zone near the surface and isolated areas of porosity in the bulk material. Magnification 200X; etchant: 3% nital.

Nitriding significantly increases the yield stress in the outer surface area of the treated component. It is estimated, based on the hardness induced in certain steels by nitriding to a level exceeding 1,000 Vickers (over 70 HRC equivalent), that the yield stress in these layers approaches 480 ksi (3,309 MPa). In many situations, nitriding is a final operation in the manufacturing cycle of the product, and as such, has to be performed with high precision guaranteed only by an ion nitriding process. Application of this process to sintered powder metallurgy (PM) products is so advantageous that no other surface technology is a comparable substitute.

Fig 2. Load of 4300 sintered PM hub synchronizers after ion nitrocarburizing

Typical ion nitriding applications

The porosity that exists in sintered products is a serious obstacle for conventional surface hardening processes like salt bath and ammonia nitriding. Salt penetrates the porous structure and has to be removed in an additional post-nitriding operation, while ammonia used in the gas nitriding process penetrates the treated part causing through-hardening and "swelling," as well as embrittlement [1-4]. Ion nitriding hardens only the surface of the product without any of the adverse effects associated with the other surface treatment technologies, and it has proved to be a reliable technique for distortion-free surface hardening of many engineering components. Advanced Heat Treat processes many automotive parts made of various PM steels and alloys using its UltraGlow(r) nitriding and nitrocarburizing treatments.

Fig 3. Photomicrograph of surface layer of Fe-0.8%P sintered low-density PM armature after ion nitrocarburizing at 1040 F (560 C). Note a compound zone near the surface and extensive porosity in the core. Magnification 400; etchant: 3% nital.

Fe-Cu-C and Fe-P steels

A typical structure of an ion nitrocarburized powder metallurgy component contains a compound zone (white layer) consisting of Fe2-3N and Fe4N type nitrides and limited porosity as shown in Fig. 1 and Fig. 3. The compound zone not only improves wear properties, but also it significantly increases corrosion resistance. Figures 2 and 4 show loads of components (PM hubs and armatures used in the automobile industry) after ion nitriding.

Fig 4. Load of 2300 PM armatures after ion nitrocarburizing

Although a compound zone is the most important element of the nitrided layer in PM sintered products made of iron-copper-carbon and similar alloys, there is always a diffusion layer below the surface with increased hardness reaching a depth of 0.010 to 0.050 in (0.25 to 1.27 mm). This layer provides good support for a much harder compound zone.

Fig 5. Cross-sectional hardness of an AISI Type 316L stainless steel PM dc rotor ion nitrided at 1080 F (580 C)

Stainless steels

The presence of a high porosity level and passivated surface layer makes stainless steel PM products very good candidates for ion nitriding. Products made of PM stainless steel do not form thick compound zones or very deep diffusion zones, but they have much higher hardness after ion nitriding than the carbon-copper-iron alloys [4]. Even a very soft annealed AISI Type 316L PM stainless steel achieves a hardness above 732 Knoop (60 HRC equivalent) after nitriding (Fig. 5).

Fig 6. Photomicrograph of the surface layer in an AISI Type 316L stainless steel PM dc rotor ion nitrided at 1080 F (580 C). Note porous nature of the alloy. Magnification 400X; etchant: Marble’s reagent.; Fig 7. Photomicrograph of the narrow slot structure in the AISI Type 310 stainless steel PM unison ring after plasma nitrocarburizing at 1080 F (580 C). Note porous nature of the alloy. White layer on the surface is a nickel coating applied for a better edge retention during metallographic sample preparation. Magnification 160X; etchant: 3% nital.

The microstructure of the nitrided layer in stainless steel contains very small nitrides and clusters, which cause the steel to etch darker (Fig. 6 and 7). Pores present in the PM steel do not affect its ability to form a consistent, uniform nitride layer, and even alloys containing very extensive porosity can be ion nitrided (Fig. 7). Figure 8 shows production ion nitriding of such parts; that is, unison rings for an automotive turbocharger made of the AISI Type 310 stainless steel PM alloy. Despite high porosity and a complex shape, the parts have a very hard. uniform nitrided layer in all of the critical areas and, at the same time, their high dimensional stability is maintained.

Fig 8. Load of 1985 unison rings after ion nitriding

Conclusions

AHT's UltraGlow ion nitriding and nitrocarburizing surface technology is well-suited for improving surface properties of engineering components made of various sintered PM ferrous alloys including stainless steels. It provides a very hard, tough surface that is resistant to abrasion and friction, as well as improved corrosion properties and fatigue strength, without affecting part dimensions.