Secondary Hardness

[Filipek144]

Hello engineers and scientists!

Let’s say that I want to achive max hardness of D2 steel after 1050°C. The easiest way is to temper it twice at ~525°C and I should get 63-64 HRC. What if I have already tempered it at 500°C and I couldn’t get more then 61 HRC and I decided to temper it then at 525°C to make it harder.
Two scenario:
a) I have tempered it only once at 500°C first and then 525°C. Will it be harder?
[I performed this scenario once and I made it harder, but it was only once and I cannot confirm my thesis.]
b) I have tempered it twice at 500°C first and then decided to temper it once more at 525°C. Will it be harder?

Basically, the question is, can I still aim to hardness range from secondary hardening after previous tempering at a lower temperature or is it too late?

Thank you!

 

[MagBen]

Double temper is often used to transform retained austenite to martensite for high carbon/alloy steels, in this sense, hardness can be increased. But why you wanted to double temper at higher temp, not same or a bit lower? temper at higher temp could also transform some martensite to ferrite, which decrease hardness.

The reason you did not get a high hardness after temper at 500C might be due to high hardening temp. Overheat may be prone to retained austenite due to depression of Ms temperature. Did you actually test hardness after 1050C hardening?

 

[Filipek144]

To be more understandable, I will describe the situation in which this question appeared.

I work at a company dealing with heat treatment. The customers always deliver us a part (or several parts) with a note which which informs what kind of material it is and what they want. These iformations are processed and on their basis is created a card, we call "job card", which is always attached to the part.
The job card always has a quality description, but sometimes it is very short.
It is very often when it says:
- material: D2
- harden at 1050C
- double temper min 500C to get 62-64 RC.

The company works 24/7 and it is very likely that one person starts heat treatment and someone another finishes it. When I see a quality description like that above, I know that I need to temper it at 525C, but unfortunetely, when unexperienced workers see 500C, they temper it at 500C, beacause they are not concious of secondary hardness peak on tempering curve.
When I take my shift, sometimes I can find a part after tempering at 500C with a note "I couldn't get more that 61RC".
When someone tempered it only once at 500C I can still get 62-64RC tempering it second time at 525C, but I couldn't get it harder when it has been already tempered twice at 500C (one attempt).
Do you have any experience with something similar? How does it look from the physical side?

 

[AidanMc]

Why don't you update the job cards to have time/temperatures/tolerances to ensure repeatable performance across shifts? If you know that it needs to be 525C to get the hardness into customer requirements, why not update the job card to say 525C ± 5C? Rather than worrying about reworking previous mistakes, wouldn't it be easier to solve the root cause of the problem?

 

AidanMc,
Agree, there is a larger issue here of continuity across shift changes.
Not an unknown problem in fabrication shops!

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[MagBen]

The general trend is the hardness will decrease as temper temp increases. How do you know 525C will give you a higher hardness than 500C? is this based on statistic data, or, just one hit?

 

[arunmrao]

Did you consider subzero treatment instead.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[EdStainless]

If a repeat temper at the same temp or a slightly higher one actually increases the hardness then there are serious problems with the HT.
If the austenitizing temp and the quench are both correct then additional tempering can only lower the hardness.

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P.E. Metallurgy, Plymouth Tube

 

Secondary hardening in D2 is a thing [URL unfurl="true"]https://www.eng-tips.com/viewthread.cfm?qid=79609[/url]

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

I have seen this in other alloys when you temper at an undesirable temperature (such as temper embrittlement or the strain hardening range).

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P.E. Metallurgy, Plymouth Tube

 

[djhurayt]

I'm afraid I am showing my ignorance here, but couldn't the OP do a normalization heat treat to get back to square one and then reprocess at the correct temps & times?