Question of the Month: For quite a while after scan hardening, we experience low hardness readings (52-55 HRC) in areas where the diameter of our 1045 steel hollow shaft changes. We have consulted with induction heating specialists, but have not eliminated the problem. What causes soft spots at these locations and how we can fix this?
Answer: Based on the hardening properties of SAE 1045 steel, it is reasonable to expect achieving a maximum as-quenched surface hardness of about 60-62 HRC. Under normal conditions, a surface hardness in the range of 52-55 HRC is lower than expected for as-quenched hardness. Generally, three groups of factors could cause the appearance of soft spots:
- Metallurgical/microstructural factors (e.g., flaws in the steel’s prior structure, severe chemical and phase segregation, presence of decarburization after previous operation, excessive amount of retained austenite, and some others)
- Inappropriate heating conditions (e.g., insufficient austenitization or formation of heterogeneous austenite, localized overheating of larger diameter shoulder or insufficient localized austenization of smaller diameter near the shoulder)
- Quench related factors, including inappropriate impingement angle and temper back effect.
Taking into consideration that hardness soft spots are associated with the diameter change area, most likely they are caused by one of the reasons related to the second or third group of factors. Due to the complexity of the electromagnetic field distribution in a diameter change region, there is always a reasonable compromise between a surplus of induced power in the shoulder of the large diameter and its deficit in the fillet or undercut of the neighboring smaller diameter area.
If lower than expected hardness readings appeared at the large diameter shoulder near the diameter change area, then most likely this is caused by the heat surplus there. This could produce excessive retained austenite, accumulate an excessive amount of heat and could also result in temper back softening.
If lower than expected hardness readings appeared at the small diameter area (e.g., fillet or undercut) of the diameter change region, then most likely this is caused by insufficient heating. The narrower heating face of the scan inductor allows for improving the heating controllability during scanning. Adjusting the scan hardening recipe and/or modifying the inductor design can help fix this problem. Therefore, if you have a handheld pyrometer, compare temperatures of larger and smaller diameters of the shoulder area to determine which of those two heat-related factors might contribute to this problem.
Shaft orientation during scan hardening can also cause lower than expected hardness readings within the diameter change region. For a shaft processed in the undesirable direction, certain areas of the shaft shoulder zone may not be sufficiently quenched out, and spray quench strokes could miss the smaller diameter located near the shoulder resulting in insufficient quenching and the appearance of soft spots. Therefore, if spray quench impingement angle (the angle at which the quenchant strikes the workpiece) is too small then its increase can help to better quench-out of the smaller diameter area near the diameter change. Keep in mind that an increase in an impingement angle should not be too drastic leading to splash back of quench into heating coil.
It should be understood that although high rotational speed can provide more uniform circumferential heating, it may also result in the deflection of the quenchant. If the rotation is too fast the quench fluid might not be able to provide proper quenching in certain areas of the shoulder resulting in soft hardness readings.
Besides, that insufficient cooling might also lead to an undesirable “tempering back,” which results in reduced hardness of the as-quenched part. Therefore, be sure there is a sufficient quench flow.
Dr. Valery Rudnev, FASM
Director, Science & Technology
Beginning in July 2016, the Professor Induction column started a new article series called “Induction Heating: Everything You Wanted to Know, But Were Afraid to Ask.” The most commonly asked questions related to different aspects of induction heating and heat treating will be reviewed and explained. All are welcome to send questions to Dr. Rudnev at email@example.com. Selected questions will be answered in this column without identifying the writer unless specific permission is granted.