AISI 1074 - Reversal of Tempered steel to As rolled condition

[shankarviyer]

We produce circular Saws out of AISI 1074 Steel. The size of the saws are 16" diameter, 0.037" thk.

The desired raw material is 1074 steel in coil form in as rolled condition with a hardness of 28 to 31 HRc.

However, we ended up with one lot of Hardened and Tempered steel strips having a microstructure of Fine Tempered Martensite and a hardness of 34 to 36 HRc. All other parameters are as desired (such as the chemistry, thickness etc). This was purely a mishap during the procurement.

Our objective is twofold:

1. Bring down the Hardness to 30HRc or closer

2. Convert the Microstructure (from fine tempered martensite to "as rolled" condition)

We are thinking of ways to heat treat these steel strips (they have been cut to the desired circular size - 16" OD x 6 7/8" ID).

One such idea is to bunch the saws together (100 to 150 ea) in a mandrel and then to normalise them, either in a vacuum furnace or in a regular normalising furnace by passing Nitrogen when the temperature reaches above 1100 Deg F (to prevent any scaling)

The cycle that we thought of is:

Heat the job lot to a temperature of 1500 Deg F - Heating @ 100 Deg F per hour

Soaking time : 30 minutes

Cool inside the furnace until 550 Deg F - Cooling @ 100 Deg F per hour

Then cool in still air outside the furnace

We are not sure, if such a situation has been experienced before by any of the members

Every suggestion is most welcome

Best

VS

 

Strictly speaking, "as rolled" condition can only be achieved by "as rolling".
Something went wrong, so the first step is to confirm the steel's composition. If that checks out, then there was an error(s) in processing.
But since it appears you have martensite, you should be able to temper this to achieve the specified hardness. It will be trial and error, so begin with tempering only one.

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

 

[Dhurjati Sen]

Dear shankarviyer,

This is a pure Physical Metallurgy question. Thanks for posting.

1. Considering the composition of the steel, 818⁰C, is a good austenitizing temperature.

2. 30 minutes required for 1 mm thick saws to be fully austenitized? Review the soaking time as the one proposed by you could lead to grain growth.

3. Why cool inside the furnace when you can take the products out at 760⁰C and cool in still air?

4. Now you get a normalized structure, with hardness levels as desired.

Still you cannot pass on the above as a rolled product as it will not have anisotropy, i.e. directional properties.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[shankarviyer]

Thanks Mr. Sen and Mr. IM

Answers to the questions of Mr. Sen:

1. About 150 saws (eac hof 1mm thick) would be clamped together in a mandrel. So the Height of the job lot would be about 150mm. Based on this, we thought of 30 min soaking time

2. We wanted to cool inside the furnace until 500 Deg C or so, to drop some hardness (from 35HRC to 30HRC) - would this be possible

3. Yes, the directional properties would be lost. We can see that. Thanks for pointing it out on the anisotropy

Answers to MR. IM:

1. We do not mind tempering one piece at a time.

2. Any suggested T-T cycle to begin with from you, please ?

Best Regards

VS

 

[MagBen]

normalize air cool
or,
austenitize again, oil quench, then, temper at high temperature (say 1050F)

no way to go back to this condition. material was probably spheriodized/annealed before cold roll, the structure is still anstenitic (elongated).

 

[shankarviyer]

Thanks, MagBen.

We would go ahead with normalising & air cool and update the results