Need input w.r.t annealing of low carbon steel rivet where hardness after heat treat be 60hrbmax

[gma301221]

Need information w.r.t annealing of 0.06% C steel rivets. Annealing at 1700F for 1hr is producing fully annealed structure with grain size number between 6 and 7.
Hardness measurement using vickers microhardness is not consistent. Need the hardness strictly max 60HRB.
Same rivet giving different hardness(73HRB and 56HRB).
Annealing time changed from 1hr to 1.5hr soaking time but no help. Please guide me through. Should I increase the annealing time at 1700F?

 

[metengr]

Before I would look at modifying your annealing time at temperature, I would be sure to check your method and accuracy for microhardness testing. Is the testing being done using a portable tester or is the rivet cross-sectioned and prepared for metallographic examination in a mount?

 

[Lyrl]

Are there visible microstructural features correlated with the lower and higher hardness indentation locations?

 

[TVP]

There is no need to increase temperature above 1700 F. How many rivets are bringer annealed at one time? How are they being cooled? I agree with metengr, make sure your testing method is accurate.

 

[gma301221]

The rivets were hot mounted and then tested crosssectionly using vickers microhardness tester at 500gmf.
rivets are 0.020"dia with 0.060" long. 1010 steel rivets heat treated at 1700F for 1,1.5hr and 5hrs soak time.
Measurement on 5 rivets after 5hours of soak time gave 3 results less than 60HRB and 2 were 67 and 69HRB out of spec.
300gmf and 100gmfloads gave higher microhardness data probably due to indentation size effects.
Chemistry of raw material is within spec. Tensile strength of wire(raw material) was reported higher than required but since we have annealed it for so long, that effect of higher tensile strength must be lost by now.
Thanks guys for all your input.
TVP: around 9000 pieces roughly are annealed at a time in a vacuum furnace and slow cooling is carried out in vaccum.
Rivets are spread out on a tray before heat treating.

 

[Lyrl]

1. Are you etching the mounts to evaluate microstructure? (I assume so based on the grain size measurement.) Are there any microstructural features associated with the softer or harder indentations?

2. Is the indentation size measured automatically or by hand? Indentation measuring software can be thrown off by all kinds of things, it is always good to double-check by an operator. And in a borderline case like you describe, I would consider it a good idea for a second person, even, to check that the measuring lines are correctly placed at the corners of the indentation.

3. Presumably you have a calibrated test block that you verify regularly? When you look over the records of your verifications with the calibrated test block, is there any tendency to readings on the high or low side of the calibrated range? This may reveal a bias on how indentations are measured (whether by software or operator).

4. What magnification is being used? Lower magnifications result in less accurate measurements.

5. Can your machine test at 1000g load? That might give you more consistent results.

 

[gma301221]

@Lyrl: Microstructure looked like annealed grains with grain size number between 5-6. After 5hours of annealing, the grains did not coarsen a lot which was surprising. I was expecting it to be in 3-4 range.
Indentation for hard ones showed more sinking in type of behavior around the indent whereas for indents within spec, indent looked normal.
We do not have test block for low hardness ranges i.e 100HV for 500gmf.The lowest we have is 241HV.
Indentation area is measured automatically. But we tried doing it manually as well and gave around similar result.
Our machine can measure at 1000gmf load but the rivets are o.02"dia with 0.06"long. So indent might crossover and give erroneous reading.

Will the test block in low hardness ranges between 90-110HV using 500gmf load be helpful in reducing the hardness inconsistancy within the same rivet.

 

[AndrewFinAustralia]

You mention Vickers Microhardness and Rockwell B

If I read correctly, you're extrapolating HRB from Vickers indents?
The rivets are small at 0.5mm dia x 1.5mm long.

Pure guess on my part, but it sounds material related (two batches?, especially if the initial hardness was high for some and grain boundaries appear to be pinned)
or hardness testing setup

If you're at a wit's end, maybe even try an anneal for 0.5hr - you may be getting segregation. After all, at a 0.5mm dia rivet, soak time needed is minimal. I'm not aware of minimum recommended soak times from standard or manufacture, so this may not be possible.

 

[Lyrl]

You should be able to use your history with the 241 HV test block to determine if your machine typically reads on the low side or high side of the calibrated range. If there is a bias of some sort it would probably show up across the board regardless of hardness.

How many indentations are you able to fit on each rivet? Two indentations too close together can make the second one artificially hard due to work hardening; the ASTM rule is to have empty space with a minimum width of two indentations between each mark.

Is it possible the parts are being work hardened by any deformation that is part of the sample prep? Grinding and polishing should be OK, but (for example) using wire cutters to make an appropriate shaped sample for mounting might introduce some work hardening.

Another thing to look at might be material. Do you have chemical analysis of the material lot? If there are any high residual elements, there might be banding (alloy segregation). Bands with higher levels of alloying elements would have higher hardness.

 

[AndrewFinAustralia]

also, gmarathe, how big is the hardness indent as a rough percentage of the diameter? Did I understand that you have mounted them in a block and are testing them transverse to drawing direction (ie: between the 'fibres' in the steel.) If I understand correctly, you are seeing a circle in the block as mounted when you do the hardness test - correct?