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.

A gas quenching method was developed by DANTE Solutions, in conjunction with the U.S. Army Combat Capabilities Development Command Aviation & Missile Center (DEVCOM AvMC), to control distortion in difficult to quench geometries. This new method addresses the nonuniform cooling inherent in most gas quenching processes. A prototype unit was constructed and tested with the aim of controlling the martensite formation rate uniformity in the component being quenched. With the ability of the DANTE Controlled Gas Quenching (DCGQ) unit to control the temperature of the quench gas entering the quench chamber, thermal and phase transformation gradients are significantly reduced. This reduction in gradients yields a more uniform phase transformation, resulting in reduced and predictable distortion. Being able to minimize and predict distortion during gas quenching, post heat treatment finishing operations can be reduced or eliminated, and as such, fatigue performance can be improved. This paper will discuss the prototype unit performance. Mechanical testing and metallographic analysis were also performed on Ferrium C64 alloy steel coupons and will be discussed. The results obtained showed that the slower cooling rate provided by the prototype did not alter the microstructure, hardness, strength, ductility, toughness, or residual stress of the alloy.

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.

This paper presents the results of a study examining the cooling rates of two vacuum high-pressure gas quenching furnaces: a large 10-bar furnace equipped with a 600-hp blower motor and a smaller 10-bar furnace with a 300-hp motor. In comparing critical cooling temperatures for H13 in the 1850°F to 1300°F range, the furnace that is almost three times larger in volume (110 vs. 40 ft 3 of hot zone) cooled the same workload almost identically to smaller unit. The test results clearly show that gas flow, or velocity, is more meaningful than pressure when it comes to cooling rate.

Rapid induction hardening of martensitic steel can attain the very high strength levels needed for light-weighting components subjected to high operating stresses. Specimens of martensitic 0.6% C steels were heat treated using a dilatometer to investigate the effects of heating rates of 5 to 500 °C/s to temperatures of 850 to 1050 °C on the transformation to austenite and subsequent transformation to martensite during quenching. Selected specimens were quenched after partial transformation to austenite to assess the initial cementite precipitate size formed in ferrite during heating. Other specimens were isothermally held at the austenitizing temperature to assess cementite dissolution rates. Higher heating rates increased the Ac1 and Ac3 temperatures, and lowered the Ms temperature. Alloy content and prior microstructure also influenced the transformation temperatures.

This paper investigates the factors that influence quenching rates and temperature distributions in steel during dilatometry testing. In a prior study, the authors assessed the performance of the cooling system in a widely used dilatometer. The goal of the current work is to develop a cooling strategy that provides more uniform and possibly faster cooling in the same system. Several alternate quench concepts are analyzed, the most promising of which uses water-cooled tubes to deliver high velocity gas through a series of jets axially aligned with the test sample. The proposed cooling apparatus and its effect on the induction heating process are assessed using CFD, electromagnetic, and thermal analyses.

This paper presents a computational approach for assessing the potential for distortion when using high pressure gas to quench steel parts. It explains how to account for component geometry, heat transfer coefficient, gas temperature and velocity, heating and cooling rates, and phase transformations. The authors employ finite element modeling methods to determine local phase fraction and displacement in a Ferrium C64 disk for different quench pressures. Simulations at timed intervals show how distortion and phase fraction progress in different areas of the disk and along the edges of an off-center bore. The causes of distortion are examined and explained using the model, with insights into why the cooling rate has a nonlinear relation with distortion.

This paper describes the inner workings of a gas quenching chamber and assesses its potential for high-volume production of precision gears. The cooling manifold in the chamber surrounds the part, which sits on a rotating table. This ensures uniform flow of cooling gas across the top, bottom, and sides of the part and achieves uniform and repeatable quenching results. In addition, because the cooling nozzles can be adjusted to fit the geometry and size of the part, distortion can be effectively controlled.

This paper reviews several recent advancements in high pressure gas quenching technology along with the impact of new higher hardenability steels. With design upgrades and improved gas flow and heat removal, a wider variety of materials, part geometries, and load sizes can now be gas quenched.