Nitriding surface hardening is commonly used on steel components for high wear, fatigue and corrosion applications. Case hardening results from white layer formation and coherent alloy nitride precipitates in the diffusion zone. This paper evaluates the microstructure development in the nitrided case and its effects on the hardness in both the white layer and the substrate for two industry nitriding materials, Nitralloy 135M and AISI 4140. Computational thermodynamic calculations were used to identify the type and amount of stable alloy nitrides precipitation and helped explain the differences in the white layer hardness, degree of porosity at the surface, and the hardening effect within the substrate. Some initial insights toward designing nitriding alloys are shown.

Low pressure carburizing (LPC) is a proven, robust case hardening process whose potential is only limited by the style and size of vacuum furnace. Today, LPC is typically used in horizontal vacuum furnaces where the opportunity to carburize large parts is limited. In this paper we present a new adaptation of the technology in large pit type vacuum furnaces, capable of opening to air at elevated temperature. This underscores the potential of LPC to carburize larger, more massive parts in a clean, effective and efficient process. The result is quality casehardened parts without the undesirable side effects of atmosphere gas carburizing such as the use of a flammable atmosphere, reduced CO and NOx emissions, no intergranular oxidation, and limited retort life. Another significant advantage is decreased process time. The case study presented here shows that eliminating furnace conditioning and increasing process temperature can significantly reduce cycle durations by nearly three times and cut utility costs in half. Under these conditions, a return on investment (ROI) is in the neighborhood of 1 – 2 years is possible, making LPC in a pit style furnace a cost-effective solution than traditional atmosphere gas carburizing technologies.

Low pressure carburizing (LPC) in combination with high-pressure gas quenching (HPGQ) is a robust and versatile case hardening technology. This paper shows how recent advancements in LPC and HPGQ are being employed in the heat treatment of automotive and aerospace components. Significant progress has been made in areas such as fixturing, load densities, cycle times, distortion control, automation, traceability, and the integration of heat treatment into manufacturing lines. Practical applications are shown for both multiple- and single-layer treatment.

The distortion behavior of carburized and fully heat treated Ni-Cr-Mo martensitic steel (S156) has been experimentally evaluated. Dimensional measurements of Navy C-ring distortion coupons during interrupted heat treatment process for parts manufactured from two forming routes, hot forging and machined from as received bar, was performed. Metallurgical analysis was carried out to attempt to relate the observed microstructural characteristics with measured process induced distortion. The carburization process was found to be the most severe in terms of inducing distortion. It was found that additional heat treatments during the process results in a larger final distortion. Machining parts from forgings results in higher distortions than that of those machined directly from as received bar due to the added thermal processing history. A finite element simulation of the carburization process for a C-ring coupon is presented.

Case hardening by carburizing is the most common heat treatment in mass production, which relies on atmosphere, or vacuum carburizing followed by oil or gas quenching and finally by tempering. Parts being heat-treated undergo the process in a configuration of a batch consists of hundreds or even thousands pieces. Under these circumstances, individual parts can’t help but be exposed to different process parameters in terms of temperature, atmosphere and quenching depends on their position within the batch. Parts near the outer portion of the load see a more rapid rise in temperature, are first exposed to the carburizing atmosphere and are more effectively quenched than parts located in the center of the batch. This can lead to significant variation from part to part and load to load; the resultant effective case depth deviation can be as high as 50%. Similarly, during quenching from hardening temperature distortion becomes highly unpredictable and unrepeatable. Modern industry demands greater precision and repeatability of results beyond those achievable by so-called traditional batch or continuous technologies and their associated equipment. Elimination of batches and focus on individual parts is the only true way to advance the industry. The article will introduce the first operational system for truly single-piece flow method for case hardening by low-pressure carburizing and hardening by high-pressure gas quench. The system treats each part individually and as such provides virtually identical process parameters, which results in extremely accurate and repeatable results. Quenching one part at a time in a specially design chamber, achieves more precise control and significantly reduces distortion so as to all make it possible to avoid post heat treatment hard machining operations. This single-piece flow heat treatment method is easily adapted into manufacturing and can be directly integrated into in-line manufacturing operations, working directly with machining centers. Materials handling and logistical issues are eliminated thus saving time and reducing unit cost. The results achieved on series of automotive gears will be reported and demonstrate incredible accuracy and repeatability, while significantly reducing distortion. Productivity and process costs prove the system to be highly competitive with other technologies. These proven advantages and savings.

Low pressure carburizing (LPC) in a vacuum furnace is increasingly the preferred method of case hardening for aerospace gears, and acetylene is often one of the gases used in the process. Selective case hardening is common with gears, where certain sections of a part are “stopped off” or “masked” to prevent carburization at those locations. For aerospace parts, the masking used is typically copper electroplating. The low pressures and high temperatures used in LPC lead to copper evaporation, which contaminates the vacuum furnace hot zone and components. In a worst-case scenario, deposited copper can lead to short-circuiting of power feedthroughs. This study looks at the effect of vacuum and partial pressure gases on copper evaporation and its application in production processes.