This article introduces some of the general sources of heat treating problems with particular emphasis on problems caused by the actual heat treating process and the significant thermal and transformation stresses within a heat treated part. It addresses the design and material factors that cause a part to fail during heat treatment. The article discusses the problems associated with heating and furnaces, quenching media, quenching stresses, hardenability, tempering, carburizing, carbonitriding, and nitriding as well as potential stainless steel problems and problems associated with nonferrous heat treatments. The processes involved in cold working of certain ferrous and nonferrous alloys are also covered.

The specially designed sand-cast low-alloy steel jaws that were implemented to stretch the wire used in prestressed concrete beams fractured. The fractures were found to be macroscale brittle and exhibited very little evidence of deformation. The surface of the jaws was disclosed by metallographic examination to be case carburized. The case was found to be martensite with small spheroidal carbides while the core consisted of martensite plus some ferrite. The fracture was revealed to be related to shrinkage porosity. Tempering was revealed to be probably limited to about 150 deg C by the hardness values (close to the maximum hardness values attainable) for the core. It was interpreted that the low tempering temperature used may have contributed to the brittleness. The procedures used for casting the jaws were recommended to be revised to eliminate the internal shrinkage porosity. Tempering at a slightly higher temperature to reduce surface and core hardness was recommended.

U-shaped leaf springs, intended to serve as spacers between oil tank floats and the inner walls of the containers, broke while being fitted, or after a short time in use, in the bend of the U. The springs were made of tempered strip steel of type C 88 with 0.84 % C, bent at room temperature, and electroplated with cadmium for protection against corrosion. Each fracture showed seven or eight kidney-shaped cracks. At the origins of these cracks on the concave inner surface of the springs, crater-like depressions and beads of melted and resolidified material were found. Fracture of the springs was caused by stress cracks as a consequence of local hardening. The hardening caused by melting and resolidification, and therefore the cracks in the springs, was the result of a faulty procedure during cadmium electroplating.

Two sand-cast low-alloy steel equalizer beams (ASTM A 148, grade 105-85) designed to distribute the load to the axles of a highway truck broke after an unreported length of service. Normal service life would have been about 805,000 km (500,000 mi) of truck operation. Investigation (visual inspection, chemical analysis, tensile testing, unetched 65x and 1% nital etched 65x magnification) supported the conclusions that the steel was too soft for the application – probably due to improper heat treatment. Fracture of the equalizer beams resulted from growth of mechanical cracks that were formed before the castings were heat treated. Recommendations included the following changes in processing: better gating and risering in the foundry to achieve sounder castings; better shakeout practice to avoid mechanical damage; better inspection to detect imperfections; and normalizing and tempering to achieve better mechanical properties.

Brass pipe couplings submitted for examination were deep-drawn from disks then annealed and subsequently cold threaded. Chemical analysis confirmed that the specified alloy Ms 63 was used for fabrication. Some of the pipe already showed fine cracks prior to their installation. In most cases however the cracks were detected after a certain period of operation. The intercrystalline course of the cracks indicated stress-cracking as it often appears in brass after heavier cold deformation. The splitting of the couplings could have been avoided by a tempering heat treatment at temperatures between 230 and 300 deg C after rolling the threads. This procedure would have reduced the internal stresses while maintaining strengthening gained by the cold deformation.

A plant had manufactured and heat treated their product in house for years. As time went on, the special steel that they had been using became more expensive, and a switch was made to a more common and less highly alloyed material. However, no change in hardness specifications were made, because calculations of ideal critical diameter and analysis of available hardenability data indicated that the original hardness specification could be met. There was, however, less room for process variation. The parts ended up containing temper carbides, developed heavy decarburization, and experienced excessive distortion because they were left in the furnace for extended and varying periods with the temperature “turned down a couple hundred degrees.”

Extra high strength zinc-coated 1080 steel welded wire was wound into seven-wire cable strands for use in aerial cables and guy wires. The wires and cable strands failed tensile, elongation, and wrap tests, with wires fracturing near welds at 2.5 to 3.5% elongation and through the welded joints in wrap tests. The welded wire was annealed by resistance heating. The wire ends had a chisel shape, produced by the use of sidecutters. Tests of the heat treatment temperatures showed that the wire near the weld area exceeded 775 deg C (1425 deg F). Metallographic examination revealed martensite present in the weld area after the heat treatment. The test failures of the AISI 1080 steel wire butt-welded joints were due to martensite produced in cooling from the welding operation that was not tempered adequately in postweld heat treatment, and to poor wire-end preparation for welding that produced poorly formed weld burrs. The postweld heat treatment was standardized on the 760 deg C (1400 deg F) transformation treatment. The chisel shape of the wire ends was abandoned in favor of flat filed ends. The wrap test was improved by adopting a hand-cranked device. Under these conditions, the welded joints withstood the tensile and wrap tests.

This article provides an overview of the effects of various material- and process-related parameters on residual stress, distortion control, cracking, and microstructure/property relationships as they relate to various types of failure. It discusses phase transformations that occur during heat treating and describes the metallurgical sources of stress and distortion during heating and cooling. The article summarizes the effect of materials and the quench-process design on distortion and cracking and details the effect of cooling characteristics on residual stress and distortion. It also provides information on the methods of minimizing distortion and tempering. The article concludes with a discussion on the effect of heat treatment processes on microstructure/property-related failures.

A precipitation-hardened stainless steel poppet valve assembly used to shut off the flow of hydrazine fuel to an auxiliary power unit was found to leak. SEM and optical micrographs revealed that the final heat treatment designed for the AM-350 bellows material rendered the AM-355 poppet susceptible to intergranular corrosive attack (IGA) from a decontaminant containing hydroxy-acetic acid. This attack provided pathways for which fluid could leak across the sealing surface in the closed condition. It was concluded that the current design is flight worthy if the poppet valve assembly passes a preflight helium pressure test. However a future design should use the same material for the poppet and bellows so that the final heat treatment will produce an assembly not susceptible to IGA.