Annealing
Annealing makes metal softer and easier to machine. You heat the part slowly, hold it at temperature, then cool it slowly. This relieves internal stresses and refines the grain structure.
Use annealing when your raw stock is too hard to machine well. It reduces tool wear and gives better surface finish.
| Material | Anneal Temp (F) | Hold Time | Cooling |
|---|---|---|---|
| 1018 Steel | 1550-1600 | 1 hr per inch | Furnace cool |
| 4140 Steel | 1525-1575 | 1 hr per inch | Furnace cool |
| 4340 Steel | 1500-1550 | 1 hr per inch | Furnace cool |
| O1 Tool Steel | 1400-1450 | 1 hr per inch | Furnace cool |
| A2 Tool Steel | 1550-1600 | 1 hr per inch | Furnace cool (40F/hr max) |
| 6061 Aluminum | 775 | 2-3 hrs | Air cool |
Hardening
Hardening makes steel much harder and stronger. You heat the steel above its critical temperature, then quench it fast in oil, water, or air. The rapid cooling locks the crystal structure into a hard state.
Not all steels can be hardened. You need carbon content above 0.3% for meaningful hardening. Low carbon steels like 1018 won't get very hard through this method.
| Steel | Harden Temp (F) | Quench | As-Quenched HRC |
|---|---|---|---|
| 1045 | 1475-1550 | Water | 50-55 |
| 4140 | 1525-1575 | Oil | 54-59 |
| 4340 | 1500-1550 | Oil | 55-60 |
| O1 | 1450-1500 | Oil | 63-65 |
| A2 | 1750-1800 | Air | 62-64 |
| D2 | 1825-1875 | Air | 62-64 |
Never use a hardened part without tempering first. As-quenched steel is extremely brittle. It can crack under impact or even from thermal shock.
Tempering
Tempering reheats hardened steel to a lower temperature. This reduces brittleness while keeping most of the hardness. The higher you temper, the softer and tougher the steel gets.
Always temper within 1-2 hours after quenching. Waiting too long can cause cracks.
| Temper Temp (F) | Resulting HRC (4140) | Best For |
|---|---|---|
| 300-400 | 52-56 | Cutting tools, dies |
| 400-600 | 48-52 | Springs, hand tools |
| 600-800 | 42-48 | Shafts, gears |
| 800-1000 | 35-42 | Structural, impact resistant |
| 1000-1200 | 28-35 | Maximum toughness |
When you specify hardness on a drawing, give a range like "48-52 HRC." A single number like "50 HRC" is impossible to hit exactly. A range gives the heat treater room to work.
Case Hardening
Case hardening makes the outside hard while keeping the inside soft and tough. This gives you the best of both worlds: a wear-resistant surface with a ductile core that won't crack.
Carburizing
Carburizing adds carbon to the surface of low-carbon steel. The part sits in a carbon-rich atmosphere at 1650-1750F for hours. Carbon soaks into the surface 0.010-0.060 inches deep. Then you quench and temper as normal.
Best for: gears, shafts, pins and bearings.
Nitriding
Nitriding diffuses nitrogen into the surface at lower temperatures (925-1050F). It produces a very hard, thin case (0.005-0.030 inches). The big advantage: parts don't need quenching, so there's almost zero distortion.
Best for: precision parts that can't tolerate distortion.
Induction Hardening
Induction hardening uses an electromagnetic coil to heat only the surface of the part. Then it's quenched with water spray. You can harden specific areas while leaving the rest soft.
Best for: shafts, pins and areas that need localized wear resistance.
Stress Relief
Stress relief removes internal stresses from machining, welding, or forming. You heat the part to 1000-1200F, hold it for 1-2 hours, then cool slowly.
Internal stresses cause parts to warp or move during final machining. Stress relief prevents this. It's common for large parts, thin-walled parts, or anything with tight tolerances.
The typical workflow: rough machine, stress relieve, then finish machine to final dimensions. This gives the best dimensional stability.
Before vs After Machining
This is the most common question in heat treatment: should you machine first or heat treat first? The answer depends on final hardness.
| Final Hardness | Approach | Why |
|---|---|---|
| Below 35 HRC | Machine, then heat treat | Soft enough to machine with carbide tools after heat treat if needed |
| 35-45 HRC | Machine, then heat treat | Standard carbide tools still work; add grinding stock on critical surfaces |
| 45-55 HRC | Heat treat, then hard mill or grind | Too hard for standard tools; need CBN or ceramic inserts |
| Above 55 HRC | Heat treat, then grind only | Only grinding can cut this hard; add 0.005-0.010" stock |
Always add 0.005-0.010 inches of grinding stock on critical surfaces before heat treatment. Parts grow and distort during heat treat. Grinding stock lets you bring everything back to spec after.
Frequently Asked Questions
Should I heat treat before or after CNC machining?
If final hardness is below 45 HRC, machine first and heat treat after. Above 45 HRC, heat treat first and finish with grinding or hard milling. Very hard parts (above 55 HRC) always need grinding after heat treatment.
What is the difference between hardening and tempering?
Hardening heats steel above its critical temperature and quenches it fast. This makes it very hard but brittle. Tempering reheats to a lower temperature to reduce brittleness. You almost always do both together.
Does heat treatment change part dimensions?
Yes. Parts typically grow 0.0005-0.002 inches per inch. They can also warp, especially thin sections. Add grinding stock on critical surfaces and machine to final size after heat treatment.
What is case hardening?
Case hardening makes the outside of a part hard while the inside stays soft and tough. Methods include carburizing, nitriding and induction hardening. It's used for gears, shafts and pins.
When should I use stress relief?
Use stress relief when your part has tight tolerances and multiple machining steps. Rough machine, stress relieve at 1000-1200F, then finish machine. This stops the part from warping during final cuts.
