maytime87
Mechanical
- Oct 23, 2014
- 5
Good evening ladies and gentlemen,
First post for me; I've have been a lurker for many years and have learned a great deal from you folks.
Background: I am a relatively new manufacturing engineer working for an aerospace Tier 1 supplier. Our tool room is making a new set of fixtures that I need to be a certain hardness. The material is a block of 4340, starting in the normalized condition from the mill. It has been waterjet cut to a certain profile, so there should be little to no residual stresses in the material. Our tool room has a small kiln and a rudimentary 30gal quench tank. We are limited to quenching with water and not oil due to regulatory requirements for ventilation hoods. We are trying to reach 40HRC. I'm not too concerned about decarb, there is plenty of excess to machine off.
I specified the following heat treat procedure;
1) Heat to 1590°F, hold for 1hr (block is about 2" x 1.5" x 14")
2) Quench in water for 20 seconds, remove and set aside until the kiln cools to 900°F for tempering. Water just at room temperature
3) Temper at 900°F for 2hrs
4) Air cool
Problem: When we programmed the kiln, we told it to shut off after 1hr of being at 1590°F. Our tool maker was about 15 minutes late taking the part out of the kiln to quench. The part was not cherry red anymore, and the oven was at about 1250°F. He also could not quench in the 30gal tank for whatever reason, all he had was a 5gal bucket of water. After quenching, he alerted me of the discrepancy. He ground the part and hardness tested various locations, and it was around 50HRC on the ends, and about 40HRC in the middle.
Before I had him temper it to draw back the hardness, I told him to wait. I researched various threads about re-heat treating steel, and found a possible treatment. I forget which thread, but the gentleman said that their SOP was to re-normalize the part (1590°F for 1hr, air cool), then reheat treat per normal (very close to my original procedure). Well, we re-normalized the part, and heated it back to 1590°F for the quench.
Once our tool maker quenched it, it cracked into two pieces! This time, he remembered to use the 30gal quench tank, water at room temp, no agitation.
Question: I know there are a lot of variables in this situation, and that our process is very far from ideal, but it is what it is. What do you all suggest we do to tighten the control of this process, and to prevent the violent cracking in the future? Should I have gone ahead with the tempering even though the quenching operation was not ideal? I have ordered an immersion heater to heat the water up to about 120-150°F, and I've fixed the agitation pump.
Thank you for your time!
First post for me; I've have been a lurker for many years and have learned a great deal from you folks.
Background: I am a relatively new manufacturing engineer working for an aerospace Tier 1 supplier. Our tool room is making a new set of fixtures that I need to be a certain hardness. The material is a block of 4340, starting in the normalized condition from the mill. It has been waterjet cut to a certain profile, so there should be little to no residual stresses in the material. Our tool room has a small kiln and a rudimentary 30gal quench tank. We are limited to quenching with water and not oil due to regulatory requirements for ventilation hoods. We are trying to reach 40HRC. I'm not too concerned about decarb, there is plenty of excess to machine off.
I specified the following heat treat procedure;
1) Heat to 1590°F, hold for 1hr (block is about 2" x 1.5" x 14")
2) Quench in water for 20 seconds, remove and set aside until the kiln cools to 900°F for tempering. Water just at room temperature
3) Temper at 900°F for 2hrs
4) Air cool
Problem: When we programmed the kiln, we told it to shut off after 1hr of being at 1590°F. Our tool maker was about 15 minutes late taking the part out of the kiln to quench. The part was not cherry red anymore, and the oven was at about 1250°F. He also could not quench in the 30gal tank for whatever reason, all he had was a 5gal bucket of water. After quenching, he alerted me of the discrepancy. He ground the part and hardness tested various locations, and it was around 50HRC on the ends, and about 40HRC in the middle.
Before I had him temper it to draw back the hardness, I told him to wait. I researched various threads about re-heat treating steel, and found a possible treatment. I forget which thread, but the gentleman said that their SOP was to re-normalize the part (1590°F for 1hr, air cool), then reheat treat per normal (very close to my original procedure). Well, we re-normalized the part, and heated it back to 1590°F for the quench.
Once our tool maker quenched it, it cracked into two pieces! This time, he remembered to use the 30gal quench tank, water at room temp, no agitation.
Question: I know there are a lot of variables in this situation, and that our process is very far from ideal, but it is what it is. What do you all suggest we do to tighten the control of this process, and to prevent the violent cracking in the future? Should I have gone ahead with the tempering even though the quenching operation was not ideal? I have ordered an immersion heater to heat the water up to about 120-150°F, and I've fixed the agitation pump.
Thank you for your time!
