Question:
We have a competition here at school and are trying to heat treat two samples of DIN 1.6582 to achieve hardness in the range of 25–33 HRC in two ways. We must produce different mechanical properties in both heat treatments. In one of the processes, we must achieve hardness in the range of 25–33 HRC while producing the following mechanical properties via tensile testing and other mechanical test methods: 830 MPa < ultimate tensile strength < 1030 MPa, 13% elongation (minimum), 55% reduction in area (minimum) and 700 MPa£yield strength. The one sample is 200 mm (8 inch) diameter x 30 mm (1 ¼ inches) high, the other sample is a pull bar whose thickest dimension is 15 mm (»5/8 inch). Please let me know how I can do this?

DIN 1.6582 is equivalent in the U.S. to SAE 4340. Remember, the size of the sample can alter the mechanical properties, especially if it is over 25 mm (1 inch). It will also affect soak times. See the important notes below as well.

Stage 1 (Goal: Achieve 25-33 HRC)
A. Heat Treatment
1. Normalize at 870°C (1600°F) soaking for one hour per inch of cross-sectional area (after the furnace has recovered setpoint and the part has reached temperature) but no less than two hours minimum.
2. Still-air cool.

B. Expected results (approximate)
1. 1280 MPa (186 ksi) Tensile Strength
2. 860 MPa (125 ksi) Yield Strength
3. 12.2% elongation
4. 36% reduction in area
5. 363 HB (»30 HRC)
6. 16 J Impact energy (Izod)

Stage 2
A. Heat Treatment
1. Harden by austenitizing at 845°C (1550°F). Hold for 30-45 minutes (after the furnace has recovered setpoint and the part has reached temperature) if the sample is 25 mm in diameter or smaller, hold for one hour per 12 mm (1/2 inch) of cross sectional area if the part is > 12 mm.
2. Quench in well agitated oil in the temperature range of 50–70°C (120–160°F)
3. Temper at 650°C (1200°F) for one hour per inch of cross-sectional area but no less than two hours (after the furnace has recovered setpoint and the part has reached temperature).

B. Expected results (approximate):
1. 1005 MPa (146 ksi) Tensile Strength
2. 938 MPa (136 ksi) Yield Strength
3. 20% elongation
4. 60% reduction in area
5. 270-285 HB (»28-30 HRC)
6. Modulus of elasticity 213 GPa
7. Endurance limit of 510 MPa (unnotched) or 138 MPa (notched).

Important Notes:

1. Protect surface of the part from oxidation or decarburization during normalizing, hardening and tempering. There are a number of ways in which to do this including the use of foil wraps (if running in a laboratory air furnace) or the use of a protective atmosphere (if running in commercial heat-treat furnaces). A protective atmosphere (typically endothermic gas) in normalizing and austenitizing must be held at a carbon potential of (approximately) 0.40-0.50% C (neutral to the carbon content of the steel, which in this case can vary between 0.37-0.43% C).
2. Temper within 15 minutes of transferring out of the furnace and oil quenching (this will avoid the possibility of quench cracking).