How to Specify Quench and Temper Cycles

Why the Quench Temper Specification Matters in an RFQ

A vague heat treatment instruction leads to failed parts. If you only tell a supplier to “quench and temper,” they will use their default cycle. That default may not match your material grade, your part geometry, or your service conditions. You need a quench temper specification that leaves no room for interpretation.

The specification must cover the material, the quench medium, the tempering parameters, and the final verification. Each of these elements controls a different part of the final microstructure. Get one wrong, and the forging may crack, warp, or fail in service.

Prerequisites Before You Write the Specification

Before you draft the document, you need to lock in the material grade and the final mechanical properties. If you are buying forged alloy steel, such as 4140, 4340, or 15-5 PH, the base chemistry and carbon content dictate the quench sensitivity. A high-carbon steel cools faster and cracks more easily than a low-carbon alloy.

You also need the final part drawings. The quench temper specification must reference the drawing number and revision. The heat treat shop needs to know the maximum section thickness, the minimum fillet radius, and any critical dimensions that must hold after cooling.

Finally, know your acceptance criteria. You need a target hardness range, a minimum toughness value, and the allowable dimensional tolerance. Without these numbers, the heat treat supplier has no way to prove they met your requirements.

Step 1: Define the Material Grade and Initial Condition

State the exact alloy grade and the condition before heat treatment. For forged parts, this is usually a normalized or annealed condition from the forging mill. Write “normalized at 1100 to 1150 C” if that is what the supplier delivers. If you are buying pre-heat-treated stock, specify the incoming condition.

Reason: The starting condition changes the grain size and the carbon distribution. A normalized forging has a different quench response than an annealed one. If the supplier normalizes at a different temperature, the grain size changes, and the quench crack risk changes.

Step 2: Specify the Quench Medium

Choose the quench medium based on the alloy grade and the part geometry. Water quenching is too fast for most alloy steels and causes severe cracking. Oil quenching is slower and gives better control. Polymer quenching sits in between. For 4140 or 4340, medium viscosity oil is common. For 15-5 PH, a polymer bath or a slow oil quench is typical.

Reason: The medium controls the cooling rate. Too fast, and the martensite transformation creates high internal stresses. Too slow, and you get a mix of martensite and pearlite, which lowers hardness and strength.

Step 3: Define Agitation and Cooling Rate

State whether the parts are agitated or still in the bath. Agitation increases the cooling rate at the surface. Still quenching is slower and more uniform. If you use agitation, specify the agitation method. Air blow, mechanical stirring, or pressure quenching all have different effects. For small parts, still quenching is often safer. For large sections, agitation helps prevent soft spots.

Reason: Agitation changes the cooling curve. A supplier that uses air agitation without you knowing will cool the part faster than your test data assumes. That can cause surface hardening and internal softness.

Step 4: Specify the Quench Soak Time

State the minimum time the part stays in the bath. For small parts, ten to thirty seconds is often enough. For large sections, you may need several minutes. Write “quench for a minimum of 30 seconds” or “quench until the bath temperature returns to the starting point.” Do not leave this blank.

Reason: The soak time ensures the core reaches the critical temperature. If the core is not fully quenched, the final hardness will be low in the center. That creates a soft core, which is a failure point in high-stress applications.

Step 5: Define the Tempering Temperature and Hold Time

State the tempering temperature in Celsius or Fahrenheit. For 4140, tempering at 580 to 650 C is common. For 4340, tempering at 570 to 620 C is typical. For 15-5 PH, tempering at 425 to 485 C is standard. Specify the hold time. One hour is often a minimum. For large parts, two to four hours may be needed for the core to reach temperature.

Reason: Tempering relieves the quench stresses and sets the final strength. The temperature controls the balance between hardness and toughness. Too high, and the hardness drops. Too low, and the part remains brittle. The hold time controls the uniformity of the transformation.

Step 6: State the Final Hardness Range and Location

Write the target hardness range in Rockwell C or HRC. State where the measurement is taken. For a flange, measure at the hub, the bore, and the face. For a shaft, measure at the center and the ends. Do not write “hardness as per drawing.” Write “HRC 36 to 40 at the hub, measured on a flat face.”

Reason: Hardness varies across a forging. The surface may be harder than the core. If you do not specify the location, the supplier may measure the easiest spot. That hides a soft core.

Step 7: Include Dimensional Tolerances and Warpage Limits

State the allowable dimensional change after heat treatment. For most forgings, a tolerance of plus or minus 0.5 percent of the nominal dimension is common. For critical parts, you may need tighter tolerances. Also state the maximum warpage. For a flange, limit the face flatness to 0.5 mm per 300 mm. For a shaft, limit the straightness to 0.5 mm per meter.

Reason: Quenching and tempering cause dimensional change. If you do not limit the warpage, the part may be dimensionally correct before heat treatment but fail after. The supplier needs to know what is acceptable so they can plan their quenching orientation and tempering method.

Step 8: Require a Heat Treatment Log and Third-Party Inspection

Demand a full heat treatment log. The log must show the furnace temperature, the quench bath temperature, the quench start and end times, the tempering temperature, and the hold time. Require a third-party inspection report. The inspector must verify the material certification, the heat treatment log, the hardness test, and the dimensional check.

Reason: The log is your proof. If the part fails, you need to know what the supplier actually did. A third-party report prevents disputes. It also catches errors that the supplier might miss.

Common Mistakes in Quench Temper Specifications

The most common mistake is to specify the hardness range but not the location. Another is to specify the tempering temperature but not the hold time. A third is to leave the quench medium open. If you do not specify the medium, the supplier will choose the one that is cheapest for them, not the one that is best for your part.

Another mistake is to ignore the part geometry. A thin section cools faster than a thick section. If you specify a quench cycle for a thin part and use it for a thick part, the thick part will crack. Always reference the drawing and note any critical sections.

Final Verification Step

Before you send the RFQ, run the specification through a checklist. Check the material grade. Check the quench medium. Check the agitation method. Check the soak time. Check the tempering temperature and hold time. Check the hardness range and location. Check the dimensional tolerances. Check the verification requirements. If any of these are missing, the specification is incomplete.

A complete quench temper specification gives the supplier clear instructions and gives you clear acceptance criteria. It reduces rework, prevents disputes, and ensures the part meets your service conditions. Write it once, get it right, and your forge will deliver the quality you need.

Table: Typical Quench Temper Parameters for Common Alloy Forgings

Alloy Grade	Quench Medium	Tempering Temperature (C)	Hardness Range (HRC)
4140	Medium oil	580 to 650	34 to 40
4340	Medium oil	570 to 620	36 to 42
15-5 PH	Polymer or oil	425 to 485	38 to 45
8630	Medium oil	580 to 650	33 to 39
316 SS	None	Solution treat then age	20 to 30 HRB

Note: These are general ranges. Always confirm with your material supplier and the heat treat shop. The exact parameters depend on the part geometry, the desired toughness, and the service conditions.

Numbered List: Key Data Points to Include in Your RFQ

1. Material grade and initial condition.
2. Drawing number and revision.
3. Quench medium type and viscosity.
4. Agitation method or still quench.
5. Quench soak time.
6. Tempering temperature and hold time.
7. Final hardness range and measurement location.
8. Dimensional tolerances and warpage limits.
9. Heat treatment log requirement.
10. Third-party inspection requirement.

Follow this list and your specification will be clear. The heat treat shop will know exactly what to do. You will know exactly what to accept. That is the goal of a quench temper specification.