Question: 1045 Steel heat treatment creation

[calliesurfer]

sample of 1045 steel, air cooled from ~800C for 15sec then water quenched. HRC of 43. Temperatures not really known. Air cooled in cool room. water was 26C intially.

My thinking: Theoretically the microstructure might be anything that lies along the time:15sec vertical line of the 1045 TTT diagram, giving almost anything there but bainite.
I guess I am looking for someone who has experiance in this- who can give me a typical air cooling temp rate, or a typical composition

thanks!!

 

[Metalguy]

You probably have ~50% martensite, 50% pearlite.

 

[metengr]

What you really need is to obtain a CCT (continuous cooling transformation) diagram of AISI Type 1045. Most of the CCT diagrams contain various cooling rate curves in deg F (C)/sec with a hardness and/or even phase transformations superimposed at the bottom of the time axis corresponding to a particular cooling rate curve. Since you started by air cooling (this will offset a normal cooling curve from 800 deg C) followed by water quenching, you can estimate which phases (ferrite, ferrite + iron carbide (pearlite), bainite and martensite) are present based on the closest cooling rate curve that results in your measured hardness.

 

[Metalguy]

As I'm sure you're aware, the problem with that method is the large hardenability differences 1045 steel-some heats/comp. harden far easier than others.

In this case we already know the hardness, but not the size.

The 15 sec. delay in water quenching is of course more important as the size decreases.

 

[calliesurfer]

specimen:
I can tell you a few more things about my processing of the specimen that will hopefully fill in a few more of the many missing pieces:
My specimen was about a 1.5" tall cylinder with a diameter of 3/4".
It was quenched in 4gal tap water (initial temp: 19.5C, final temp:26C). Afterwards, about 1/2" was sliced off (rather roughly) of one end after the heat treatment, the remaining expose circular face was polished .3u, then .1u cloth grit.
The HRC values were obtained on one of the faces. 43.8 was an average of many very similar values that were found running long the diameter. There was no significant change in the hardness between the inside and the edges.
Some of the grooves left from the blade were too deep to remove by polishing- they show up in parts of the SEM pictures.
The surface was chemically etched with 2% Nital and then the pictures were taken. (x200, x400)

 

[Metalguy]

What pictures?

 

[49078]

Sounds just like a lab I did in school...I no longer have my notes though.

 

[dbooker630]

Was the part sectioned with a band saw or a wet cutoff wheel? You mentioned that it was cut rather roughly so I would check for evidence of overheating effect on the microstructure. As Metalguy said earlier we have no pictures to see.

 

[redpicker]

For a 0.45% C steel, an as-quenched hardness of 43 HRC represents less than 50%. At 0.5" from the end, you may not be past the "end effect" (usually assumed to affect 1 radius). If you are past the "end effect", you should be seeing a hardness drop from surface to center. The fact that you have not fully removed the tool marks from the cutting operation suggests you may be picking up some effect from the cutting operation on the hardnesses you are obtaining. And, on the issue of cutting, you have to be careful with the cutting operation on as-quenched material to keep the temperature down. Temperatures above 200 deg F (or, 100C) can affect the hardenss.

All that being said, I am not sure what your question is. 800 C (1475 F) is marginally sufficient for austenitizing 1045. If anything happened during the air cool portion of the HT cycle, you should see free ferrite in the microstructure. In the AQ condition, I'd expect martensite and "a mix of higher temperature transformation products" meaning a mixture of bainite, fine pearlite, and possibly primary ferrite (depending on the actual composition, temperatures, and cooling rates.