Machine Design #63: Heat Treatment and Hardness — Do Not Stop at One HRC Number
1. Start with function and evidence
Before choosing a dimension or opening a supplier catalog, write what the heat-treated part must do, where the load or environment comes from, how many cycles it must survive, and what counts as failure. “Use the old drawing” hides assumptions about process, access, maintenance, and safety. A testable requirement states the input condition, operating condition, expected result, acceptance limit, and measurement method. Internal standards are a starting point; the final decision must be checked against the real model, material, process, and installation.
2. Review four layers together
Function and load
Separate nominal, start-up, impact, misalignment, and fault cases. Trace the load through the complete assembly, not only the attractive CAD section. Include inertia, thermal movement, cleaning, vibration, and the number of cycles that actually matters. State the boundary and the reason for the safety margin.
Material and manufacturing
The same geometry behaves differently after cutting, forming, welding, machining, coating, or heat treatment. The drawing must describe what a workshop can make and measure. A tight tolerance cannot compensate for an unknown process, an unprotected datum, or an inspection method that cannot reach the feature.
Interfaces and assembly
Define datums, direction, clearance, contact, fastening, tool access, and mistake-proofing. If variants exist, use geometry or markings to prevent a wrong part or orientation. Record the force, sequence, and evidence expected from the operator.
Operation and maintenance
Check access, replacement time, re-location, cleaning, adjustment, and the measurement that returns the machine to baseline. A design that works only with one experienced fitter is not repeatable.
3. Core checks for this topic
- Base material, condition, and required mechanical property: state the base material and its starting condition together with the property you actually need — wear resistance, core toughness or dimensional stability.
- Hardness value, scale, location, and effective depth: give the hardness value with its scale, the location it applies to, and the effective case depth; a surface number alone does not describe a case-hardened part.
- Quench, temper, case-hardening, or other process route: name the process route, since quenching, tempering and case hardening each carry a different distortion and a different cost.
- Distortion allowance, machining sequence, and datum protection: allow for distortion in the machining sequence, and protect the datum surfaces that later operations depend on.
- Residual stress, cracking, embrittlement, and surface condition: consider residual stress, cracking risk, embrittlement and the surface condition after treatment, especially on parts that are plated afterwards.
- Test method, sample location, lot traceability, and acceptance: define the test method, where the sample is taken, how the lot is traced, and what counts as acceptance.
The checklist is useful only when every line has evidence. “Reviewed” is not “calculated”, and “calculated” is not “tested at the boundary”. Keep an evidence ID next to each requirement so an engineering change can be audited.
3b. What heat treatment does, and where on the part it does it
"Heat treatment" is not one operation but a family of them, each solving a different problem. Choosing the wrong one leaves the part either not hard enough where it matters, or distorted after processing.
| Family | What it does | Suits |
|---|
| Through hardening and tempering | Heats to the austenitising range, quenches to gain hardness, then tempers to reduce brittleness | Parts that must be hard throughout: punches, dies, cold cutting tools, gauges |
| Nitriding | Creates a hard layer on the steel surface, usually with small distortion | Sliding contact faces, parts needing anti-galling, parts already close to finished |
| Repeated high-temperature tempering | Stabilises the structure of high-speed steel | SKH51 and the heat-resisting group |
For nitriding, the names used in practice are many and not interchangeable: gas nitriding gives a relatively deep case but takes longer and costs more; gas soft nitriding (ガス軟窒化, also called SN 処理) is faster and cheaper with a shallower case; salt bath soft nitriding (Tufftride) is short in duration, reasonable in cost, and leaves a dark grey surface. Writing only "nitriding" hands the choice to the processor.
One point often misunderstood: a very hard surface does not mean a high load capacity. Case depth and the contact conditions have to be considered as well. If the part sees high contact pressure, impact loading or heavy line contact, a thin hard case can sink together with the substrate beneath it.
Harder is not always better
With tool steel there is always a trade-off between hardness (wear resistance) and toughness (chip resistance): tempering at a low temperature gives the highest hardness but more brittleness, while tempering higher gives up a little hardness for more toughness.
| Working condition | Direction for the target hardness |
|---|
| Pure abrasion, little impact | Keep hardness high |
| Abrasion plus impact and shock loading | Lower the target hardness somewhat for toughness |
| Impact is the governing constraint | Consider a shock-resisting tool steel grade and accept lower wear resistance |
A press punch left too hard resists wear well but chips the moment it meets a hard blank or runs off-centre. The skill is choosing the hardness that is just enough for the real load, not choosing the largest number.
The operation sequence decides whether the part ends up the right size
Steel in the hardened condition is difficult to cut with ordinary tooling, and the hardening process itself distorts the part. So the standard sequence is:
- Rough machine while the steel is soft, leaving stock.
- Harden and temper.
- Grind the datum faces.
- Wire cut or grind the precision profile.
The stock left must be enough to absorb the hardening distortion while still leaving material for the next step, but not so much that it wastes time, since grinding and wire cutting are far slower than milling. Finishing before hardening is the classic mistake: the part comes out of the furnace no longer the right size.
On the drawing: one hardness number is not enough
Writing "HRC60" alone leaves four questions open. The drawing should state:
- Which zone has to be hard, and which zone must be masked or left soft.
- The hardness tolerance, in a form such as 60±2 HRC rather than a single absolute value.
- The scale and the measuring position — on which face, after which operation.
- The case depth if it is a surface treatment, because surface hardness alone does not describe
load capacity.
With nitriding, masking a zone that must not be treated is possible but the zone has to be stated and agreed with the processor from the start, not discussed after the parts come back.
See also Materials #12 — SKD11 and SKH51 and Materials #05 — Nitriding.
4. Tolerance stack and variation
Build the stack from the functional datum to the characteristic that must be protected. Separate nominal, worst-case, and statistical values only when the process is stable enough to justify it. Include flatness, squareness, coating or treatment thickness, clamping deformation, temperature, field assembly error, and wear.
If assembly succeeds only because a technician nudges a part, the design has no reliable capability. Assign each contributor a source and identify whether it is controlled by the supplier or verified at incoming inspection. For heat-treated part, pay particular attention to material condition, hardness depth, distortion, residual stress, and traceable testing.
5. Failure modes before release
Ask: “If this is wrong, what will the machine show, and what evidence will reveal the cause?”
| Failure mode | Machine symptom | Verification direction |
|---|
| Boundary selected too optimistically | Passes a demo, fails at speed, temperature, load, or cleaning | Test min/max and the defined fault case |
| Tolerance not tied to function | Difficult assembly, play, bind, noise, or leak | Rebuild the stack from functional datums |
| Manufacturing step omitted | Distortion, burr, crack, drift, or poor finish | Review the process and measure after each critical step |
| Inspection cannot reach feature | Supplier report looks complete but function is unknown | Define a reachable method and a sample plan |
| Maintenance not designed | Long replacement or wrong restoration | Run a maintenance trial and restoration check |
| Documents out of sync | Correct name but wrong revision or setting | Baseline BOM, drawing, process, and work instruction |
6. Drawing and record discipline
Specify only requirements that have functional meaning and can be inspected. For a special requirement state the scope, datum or measurement location, condition, and acceptance limit. Notes such as “accurate machining” or “assemble carefully” are not instructions. Keep the calculation or rationale, source revision, assumptions and limits, review comments, prototype or FAT result, and the point to recheck after change.
7. Design review on the real production route
Start at the input and follow the load or environmental attack to the output. Name the surface, edge, joint, thread, hole, treatment, or contact that carries the function. Mark where friction, clearance, temperature, chemicals, operator adjustment, or cleaning can change the result. If a parameter is unknown, assign an owner and measurement plan instead of hiding it in a large factor.
Ask the fabricator which operation creates the most variation, how the feature is inspected, and what happens after deburring, welding, heat treatment, coating, or washing. Compare capability with the stack. If the process cannot hold the drawing, change the process or design before ordering.
On the assembled machine, run start-up, normal duty, stop, restart, and a controlled fault. Record force or torque, temperature, vibration, noise, motion, visual marks, and replacement time as relevant. These observations often reveal an assumption that a drawing review missed.
8. Boundary conditions and calculations
Write lower and upper values for every influential parameter and include combinations that can occur together. Keep units consistent, show the equation or reference, and record catalog test conditions and correction factors. Compare at least one analysis result with a hand calculation or a measured prototype.
For heat-treated part, a useful sheet has columns for input, nominal, lower bound, upper bound, source, result, pass/fail, and “what would invalidate this assumption?” This makes the decision robust when a supplier changes material, cycle time, temperature, chemical, or inspection route.
9. Supplier and incoming inspection
The supplier package should include drawing revision, material condition, special process, inspection points, sample record, and rework rule. Do not outsource design intent. If a supplier proposes a different process, compare function, durability, cost, lead time, and inspection capability before approval.
At incoming inspection, measure the features that protect the function rather than every dimension with equal effort. Record actual values and instrument ID. A part can be within a drawing tolerance and still fail because of the wrong surface, burr, fit, treatment, lubricant, or orientation. Link the result to the lot or serial used on the machine.
10. Commissioning and maintenance feedback
Create a baseline at commissioning: cycle time, force or torque, temperature, vibration, noise, position, surface condition, or appearance as relevant. Define the alarm limit and the action when it is reached. After the first service interval, compare the trend with baseline and update the maintenance instruction. Replacement should restore the baseline, not merely make the machine move again.
Heat treatment changes the size: do not write a hardness and forget the rest
Heat treatment does more than raise hardness; it changes the size and can bow the part through phase change and rapid quenching. A drawing that only says "harden to HRC X" without accounting for that produces a hard part that is the wrong size.
Several things must be decided together:
- Grind allowance after heat treatment. Precision surfaces should be finished after hardening, so leave stock and state which dimensions apply "after heat treatment".
- Separate surface hardness, core hardness, and case depth. Case hardening gives a wear-resistant hard skin over a tough core; state the surface hardness, the core hardness, the case depth, and the measurement location separately.
- Quench distortion. Rapid quenching bows parts; thin or long parts warp easily. Consider press/fixture quenching, symmetric layout, and a flatness tolerance after heat treatment.
- Surface decarburization softens the outer skin; if the surface carries fatigue load, remove that layer or protect the surface during heating.
| Omission | Consequence | Correct note |
|---|
| Only HRC, no allowance | Wrong size after quench | State "after heat treatment" dimensions plus grind allowance |
| No split of surface/core/depth | The heat-treater guesses | State surface hardness, core hardness, case depth, and measurement location |
| Long/thin part | Bows after quench | Consider press/fixture quench, allow a flatness tolerance after heat treatment |
11. MINATA release checklist
- [ ] Function, load path, duty cycle, and fault or environment boundary are written.
- [ ] Material, process, surface condition, and inspection method are agreed.
- [ ] Functional datums and tolerance stack are visible.
- [ ] The six topic checks above have evidence and pass/fail criteria.
- [ ] Assembly, tool access, orientation, and mistake-proofing were tried on hardware.
- [ ] Failure modes have an owner, evidence, and re-test condition.
- [ ] Supplier and incoming records link to the revision and lot.
- [ ] Commissioning baseline and maintenance response are defined.
Frequently asked questions
What hardness should be specified?
There is no universal number. The target hardness has to balance against the real load: keep it high for pure abrasion, lower it for toughness where there is impact. And it must carry a tolerance, for example 60±2 HRC, together with the measuring position, because "HRC60" alone does not tell the shop where to stop tempering.
How does nitriding differ from through hardening?
Through hardening hardens the whole section and causes noticeable distortion; nitriding creates a hard surface layer with usually small distortion, which suits parts already close to finished or parts needing an anti-galling sliding face. In exchange, the nitrided case is thin and cannot carry high contact pressure.
Why does a part bow after hardening?
Because hardening heats and then rapidly cools the part, generating stress and distortion — especially on large or asymmetric parts. The remedy is not a tighter tolerance but leaving stock and reserving the precision surfaces for grinding or wire cutting after hardening.
Can a zone be masked from treatment?
It can, but the zone has to be stated on the drawing and agreed with the processor before the parts are sent. Masking is manual work with a cost, so grouping the zones that need to be hard is cheaper.
Can a part be machined after heat treatment?
It can but it is expensive: hardened steel has to be ground or wire cut rather than milled. That is why the operation sequence has to be planned during design — rough while soft, leave the precision profile and the datum faces for after hardening — not decided once the drawing is out.
12. Closing note
Good mechanical design is a chain of explicit assumptions that survives manufacturing, assembly, operation, maintenance, and change. For heat-treated part, the right question is not only “will it work?” but “what evidence will show it continues to work when the boundary moves?” That is the standard MINATA uses to turn a drawing into a dependable machine.
Frequently asked questions, continued
Why is a part in size before hardening but out of size after?
Because the phase change during heat treatment makes the material grow or shrink, and the rapid quench distorts it. Precision surfaces should be finished after heat treatment with a planned grind allowance; the drawing must say which dimensions apply after the quench.
Is "HRC 58" for the whole part correct?
Usually not. Many parts need a hard surface with a tough core: state the surface hardness, the core hardness, and the case depth separately, with the measurement location. A single number for the whole section is easy to misread and hard to accept.
Conclusion
Heat Treatment and Hardness is not a matter of picking a single number. Good design ties function, load, material, process, tolerance, assembly and maintenance into one contract that can be checked. An internal standard preserves the experience already accumulated; the designer's job is to understand the conditions under which it applies and turn it into a clear decision on the product at hand.
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