Do Multiple Temper Cycles Soften Steel? 1

[mewhg]

Let's say a steel component is tempered from full hard back to a certian hardness by tempering at xxxF for 1 hour.

The part is allowed to cool and then the xxxF cycle is repeated perhaps several times.

Would the part become progressivly softer?

The steel we are working with is 5160 at a target hardness of 44Rc.

The question has come up as some parts were found to be below spec on hardness and we are thinking that the parts may have been run through the tempering furnace several times.

many thanks

 

[swall]

Multiple tempers at the original temperature will not soften the part. It takes really long times (days) for the tempering to lower the hardness below that obtained after a standard one hour temper cycle.

 

[Carburize]

Multiple tempering at the same temperature will reduce hardness particularly for materials in the hardness range 45 - 60Rc. Each cycle will produce less reduction but there will be an effect which is why parts which are thermally treated after tempering, for example by post weld heat treatment or nitriding, are usually controlled to ensure that subsequent treatments are at least 50 degrees F and sometimes 100 degrees F below the tempering temperature.

 

[Maui]

There could be a number of reasons why the hardness of the part is below your specification limit. In order to isolate the root cause of the problem, you will need to review the heat treatmnent procedure that was used. Verify that the austenitizing temperature, time at temperature, and quenching medium and procedure were performed to spec. Also determine the type of heat treating furnace that was used (i.e. vacuum, controlled atmosphere, etc.) Also determine the tempering temperature that was used, the time at temperature, and the number of tempers.

Rapidly quenching the part transforms the austenitic structure to a martensitic structure, and in the as-quenched condition freshly formed martensite is extremely brittle. The purpose in tempering the part is to soften the martensite that is formed during the quenching procedure. Since the part will also have some amount of retained austenite after quenching, the first temper will also transform some amount of this retained austenite to untempered martensite. The second temper softens this freshly transformed martensite, but some amount of the remaining retained austenite will then transform to martensite, and so you can see why multiple tempers are required. And yes, mutiple tempers will reduce the hardness of the part for this reason. But the degree to which the part softens is a strong function of the tempering temperature that is used.

Maui

 

[swall]

Well, I should have qualified my answer. If your parts were given a one hour temper at say 850F, which is within expections for 5160 in your hardness range, you might lose two Rockwell points if the parts were subsequently tempered at 850F, say four more times. So, unless the parts were at the very bottom of the hardness spec range, several retempers should not have caused them to drop out of spec, given that a 5 point Rockwell spread is common for heat treat specs.

 

[NickE]

Just as an aside to the multiple tempering question. (And an addition to Maui's excellent post).

Some steels require additional tempering cycles to have sufficient toughness for their required service.