Heat Treatment Tolerances and Full Technical Requirements for CNC Parts

• 1
  What heat treatment tolerances actually control
• 2
  Why quenching and tempering move the dimensions
• 3
  Case depth and surface hardness limits
• 4
  Section thickness, mass, and hardenability
• 5
  Writing heat treatment requirements on a drawing
• 6
  How hardness and case depth are verified
• 7
  Heat treatment processes and what they hold

A hardness callout on a drawing is a range, not a number. HRC 50-55 means the part must land somewhere inside a five-point window after tempering. That window exists because the same alloy, quenched from the same furnace, will not read identically at every point on the part. Surface decarburization, section thickness, and quench agitation all move the result.

Engineers often treat heat treatment tolerances as a property of the material. They are really a property of the process. A 4140 shaft quenched in oil and a 4140 shaft quenched in polymer will not reach the same hardness, even though the chemistry is identical. The cooling rate decides the microstructure, and the microstructure decides the number a hardness tester reports.

Three quantities matter on most drawings. Core hardness sets load capacity. Case depth sets wear life on a contact surface. Distortion allowance sets how much material the grinder must remove after quench. Get any one of them wrong and the part either fails in service or fails at final inspection, before it ever ships.

So the practical question is not "what hardness do I want?" It is "what hardness window can the process hold on this geometry, in this alloy, at this section size?" Answer that first, and the tolerance follows.

• 1
  Core hardnessLoad capacity and fatigue strength of the bulk section
• 2
  Case depthWear resistance at the contact surface
• 3
  Distortion allowanceStock left for grinding after quench
• 4
  Hardness uniformitySpread between surface and mid-section readings

Steel expands when it is heated and contracts when it cools, but the transformation from austenite to martensite does the opposite: it grows. Martensite occupies more volume than the austenite it came from. On a part with uniform section, that growth is roughly even and predictable. On a part with a thick hub next to a thin web, the two regions cool at different rates and grow by different amounts.

That mismatch is distortion. It shows up as bow, twist, ovality, or a change in length that no one planned for. A 300 mm shaft can bend 0.15-0.30 mm through a water quench when the section is not symmetric. Oil quenching cuts that roughly in half because the cooling is slower and gentler.

Tempering follows the quench and does two things. It relieves a large part of the internal stress, which reduces the risk of cracking during grinding. It also drops the hardness to the target range. Higher tempering temperature means lower hardness and better toughness, so the two properties trade against each other.

The order of operations matters here. Machining to final size before quench usually fails, because the quench will move the dimensions outside the drawing tolerance. The proven route is to leave grinding stock, harden, then finish. How much stock depends on section and quench severity.

• 1
  Rough machineLeave 0.2-0.5 mm per ground face
• 2
  Harden and temperReach the target HRC window
• 3
  Stress reliefOptional second temper before grinding
• 4
  Finish grindHold final tolerance after all growth

Induction hardening and carburizing create a hard skin over a tough core. The case depth is measured from the surface to a defined hardness level, often HRC 50 or half the surface value. A typical induction-hardened shaft carries a case of 0.5-2.0 mm. Carburized gears run deeper, commonly 0.8-1.5 mm on module 2-4 teeth.

Depth is set by time and temperature, not by the alloy alone. Doubling the heating time at the same power roughly doubles the depth, but it also coarsens the grain near the surface. That trade is why induction coils are shaped to the part: the coil controls where the heat lands, and the dwell controls how deep it goes.

Too shallow a case and the surface will not carry the contact load. Too deep and the part behaves like through-hardened steel, losing the tough core that resists crack growth under bending. On a gear tooth, an over-deep case is a real risk: the whole tooth can harden through and become brittle.

Surface hardness after hardening is usually specified as a minimum, not a range, for induction work. HRC 55 minimum on a 1045 shaft is common. Because the reading is taken on a curved surface, the tester must use the right anvil and often a correction factor, or the number will read low.

• 1
  0.5-1.0 mmSmall shafts, light contact load
• 2
  1.0-2.0 mmGeneral shafts and pins
• 3
  0.8-1.5 mmGears, module 2-4
• 4
  2.0-4.0 mmHeavy rolls, large bearings

Hardenability is the depth to which an alloy can harden under a given quench, and it is measured by the Jominy end-quench test. Two steels with the same carbon content can have very different hardenability once you add chromium, molybdenum, or nickel. That is why 4140 hardens deeper than 1045, and why 4340 hardens deeper still.

The practical rule is simple. Thin sections cool fast and harden easily. Thick sections cool slowly and may not reach the target at the core. A 60 mm diameter 1045 bar quenched in oil will show a soft core even when the surface hits HRC 55. If the drawing asks for HRC 50 throughout, the alloy or the quench has to change.

Mild steel like 1018 cannot be through-hardened to any useful level. It can be carburized, which is why low-carbon steel is the normal choice for case-hardened parts. Aluminum and most stainless grades do not respond to quench-and-temper at all, apart from the 400-series and 17-4PH precipitation-hardening grades.

This is where the drawing and the shop often disagree. The drawing says HRC 50-55 on a 50 mm section of 1045. The shop knows that is not achievable through the section and proposes 4140 instead. The right answer is to accept the alloy change, not to argue about the furnace.

• 1
  1045Through-hardens only in thin sections
• 2
  4140 / 4340Deeper hardening, better core properties
• 3
  1018Carburize only, no through-hardening
• 4
  17-4PHPrecipitation hardening, H900 to H1150

A heat treatment note should answer four questions: what process, what hardness, where, and what happens to the dimensions. A note that says only "harden to HRC 50-55" leaves the shop guessing on three of them.

Name the process. "Through harden and temper" and "induction harden" produce different parts. If only a local zone is hardened, mark that zone on the drawing with a dimension or a hatched area, and state whether the transition is allowed to be gradual or must be sharp.

State the hardness and the method. HRC, HV, and HRB are not interchangeable. For thin walls under 3 mm, use HV or HR15N instead of HRC, because the HRC indenter punches through the case and reads the core. This single change prevents a lot of argument at incoming inspection.

Finally, state the post-heat-treatment condition. If the drawing tolerance is ±0.02 mm and the part is quenched, the shop needs to know whether it may grind after hardening or whether the surface is a finished, coated face. Add a note like "allow grinding stock on all diameters" and the process becomes predictable.

• 1
  ProcessThrough harden, carburize, induction, nitr ide
• 2
  Hardness and scaleHRC, HV, or HR15N with a range
• 3
  LocationWhole part or a marked zone
• 4
  Post-HT conditionGrind stock, straighten, and finish

Hardness is checked with a benchtop tester on a cut and mounted sample, or with a portable tester on the finished part. The benchtop reading is the reference; portable readings are corrected against it. For HRC, the standard test load is 150 kgf, which limits use to sections thick enough to take the indentation without bulging.

Case depth is read on a polished cross-section under a microhardness tester, taking readings every 0.05-0.1 mm from the surface inward. The depth is where the curve crosses the defined hardness level. This is a destructive test, so it runs on a sectioned sample or a test coupon, not on every part.

Distortion is checked with a dial indicator on centers, or on a granite plate for flatness. If the drawing calls out a straightness value, that check belongs on the inspection plan after hardening, not before. A part that was straight before the quench is not evidence of anything.

For production runs, the sensible pattern is a first-article check that covers hardness, case depth, and distortion, then periodic checks during the run. That keeps the process honest without cutting up parts that the customer paid for.

• 1
  Benchtop HRC150 kgf load, reference reading
• 2
  Microhardness traverseCase depth on a polished section
• 3
  Dial indicatorBow and runout after quench
• 4
  First articleFull check, then periodic sampling