Low pressure carbonitriding and pressurized gas quenching heat treatments were conducted on four steel alloys. Bending fatigue tests were performed, and the highest endurance limit was attained by 20MnCr5+B, followed by 20MnCr5, SAE 8620+Nb, and SAE 8620. The differences in fatigue endurance limit occurred despite similar case depths and surface hardness between alloys. Low magnitude tensile residual stresses were measured near the surface in all conditions. Additionally, nonmartensitic transformation products (NMTPs) were observed to various extents near the surface. However, there were no differences in retained austenite profiles, and retained austenite was mostly stable against deformation-induced transformation to martensite during fatigue testing, contrasting some studies on carburized steels. The results suggest that the observed difference in fatigue lives is due to differences in chemical composition and prior austenite grain size. Alloys containing B and Nb had refined prior austenite grain sizes compared to their counterparts in each alloy class.

Low pressure carbonitriding (LPCN) has the potential to improve the impact and fatigue strength of steel components through the enrichment of nitrogen and the effect of carburizing at higher temperatures. The work described in this paper investigates the influence of boron on the LPCN response of 20MnCr5 steel and the effect of niobium on that of 8620. LPCN treatments were developed to achieve a surface hardness of ~700 HV and case depth of 0.65-0.75 mm in four alloys: 20MnCr5, 20MnCr5 + B, 8620, and 8620 + Nb. The hardness and case microstructure of treated and quenched test samples are correlated with bending fatigue measured in Brugger fatigue specimens, which simulate the root of a gear tooth.

This presentation will discuss data on parts tested in Low Pressure Carburizing using oil and gas quenching. We will present data on metallurgy, distortion and load design to optimize each quenching media. As we know oil and gas quench respond differently, we will explore the evolution of high pressure gas quenching as it exist in today’s market. Low Pressure Carburizing has been growing among OEM’s and now Tier 2 suppliers as well as heat treaters in the Automotive and Aerospace markets. These details should help show the audience that they also can take advantage of the clean environment from Low Pressure Carburizing and just in time processing along with possible distortion control for all their parts currently being atmosphere carburized.

For more than 20 years now Low Pressure Vacuum Carburizing (LPC) has taken over several industries as the main carburizing choice. These industries take advantage of LPC’s clean environment, versatility, “just in time” processing, along with possible distortion control that comes with gas quenching capabilities to process millions of parts each month. However, there are still hundreds of types of parts that can be converted to LPC with minimal effort. The authors will show the recent history and products currently being processed in LPC and how they were transitioned to LPC. Metallurgical results will be shown along with production loading scenarios. In addition, the process of a particular part showing timing and cost of each treatment process will be reviewed. These facts will show the audience the benefits and how they can take advantage of low pressure vacuum carburizing.