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Metallurgical Engineering DISCUSSION
A shaft in 4140-type steel was supplied quenched and tempered at about 32 HRC. During a repair, the area around a seized bearing was heated with a torch to get the bearing off, and part of the shaft turned blue and then dull grey. A hardness check near that area now reads lower than the rest.
How hot can a quenched and tempered steel get before its properties change, does the time at temperature matter, and is the shaft recoverable?
The reference point is the original tempering temperature. Tempered martensite is stable as long as it stays below the temperature it was tempered at; heat it above that and it tempers further and softens. A 4140-type steel at around 32 HRC has typically been tempered somewhere around 500 to 600 °C, though the exact figure depends on the section and the heat treater, so the safe limit for a part like yours is a good margin below that.
A part tempered at only 200 °C to keep high hardness is far more sensitive: a tool or bearing race will lose hardness if it is taken to 300 °C.
The second boundary is the lower critical temperature, about 727 °C for plain carbon steel and a little different for alloy steels. Above it the structure starts to transform to austenite, and what you get afterwards depends on how it cools. Slow cooling leaves soft ferrite and pearlite. Fast cooling, for instance a thick shaft pulling heat out of a small torch-heated zone, can leave fresh untempered martensite, which is hard and brittle.
So an overheated zone can contain both a softened band and a hard, crack-prone band next to it, the same pattern as the heat-affected zone of a weld.
Time matters, but much less than temperature. The two are combined in the Hollomon-Jaffe parameter P = T × (C + log t), with T in kelvin, t in hours and C about 20 for many steels. Equal P means roughly equal tempering effect.
Example: 1 hour at 600 °C gives P = 873 × 20 = 17,460. To reach the same P at 550 °C needs 20 + log t = 17,460 / 823 = 21.2, so t is about 16 hours. A drop of 50 °C is worth a factor of 16 in time, which is why a few minutes of torch heat well above the tempering temperature does real damage while hours slightly below it do little.
Temper colours give a rough idea of what the surface saw. On clean plain carbon steel, pale straw appears around 220 to 240 °C, purple around 270 to 280 °C and blue near 300 °C; above roughly 350 °C the colours fade to grey. They depend on time and on alloy content, so they are only indicative. Blue alone would not soften a part tempered above 500 °C, but dull grey means it went well beyond the colour range, and your lower hardness reading says it passed the tempering temperature locally.
Map the zone with a portable hardness tester at close spacing so you know its extent.
Whether it is recoverable depends on where the softened zone is and what the shaft does. Under a bearing seat on a lightly loaded shaft, a modest local drop may be acceptable; at a highly stressed fillet or keyway it is not, because fatigue strength follows hardness. The sound repair is to re-heat-treat the whole shaft (austenitise, quench and temper), which brings distortion and needs finish-machining allowance, or to replace it.
Next time, remove the bearing with a puller or an induction heater aimed at the bearing, or cut the race off. Temperature-indicating crayons are a cheap way to make sure a part stays under a chosen limit.
