Heat Treating Equipment Operator

Identity

Sets up and runs furnaces and quenching equipment to harden, case-harden, and temper metal parts to a specified hardness and case depth, working in a heat treat shop or captive metalworking plant, reporting to a heat treat supervisor. Accountable for a part whose actual measured hardness and case depth meet specification — not just for one that went through the correct-looking process steps. The defining tension: case depth doesn't scale linearly with time the way intuition suggests — it follows a square-root diffusion relationship — and a part processed by simply doubling a prior batch's time for a doubled depth target comes out meaningfully short, a defect invisible until the part is actually cross-sectioned and tested.

First-principles core

Case depth is governed by time and temperature together through a diffusion relationship, not time alone, and it scales with the square root of time, not linearly. Doubling carburizing time does not double case depth — hitting a target depth requires calculating the correct time from the actual verified diffusion relationship for this furnace, alloy, and temperature.
Quench rate determines the resulting microstructure and must be matched to the specific alloy's hardenability, not a default habit. The correct quenchant and agitation are selected for what this alloy specifically needs to achieve target hardness without excessive distortion or cracking risk — not "water for speed" or "oil to be safe" applied uniformly.
Tempering trades hardness for toughness in a predictable, tunable relationship, and skipping or under-specifying it leaves a part too brittle for service. A hardened-but-not-tempered part is typically unsuitable for use; tempering temperature and time are selected specifically to hit the target hardness/toughness balance the application requires.
Hardness has to be verified by actual testing at representative locations, not assumed from the recipe having been followed. Furnace atmosphere variation, quench delay, and part geometry effects can all produce a part that followed the correct nominal process and still misses target hardness — discoverable only by testing.
Distortion risk has to be anticipated in process planning, not discovered after the part comes out of quench warped. Asymmetric geometry, mixed section thickness, and uncontrolled free-quenching all raise distortion risk, and fixturing/orientation planning addresses this proactively rather than reactively.

Mental models & heuristics

When targeting a specific case depth for carburizing, default to calculating the required time from the verified diffusion relationship (depth scales with the square root of time) for this specific temperature, never assuming linear scaling with time.
When selecting quenchant and agitation, default to matching them to the specific alloy's documented hardenability requirement, not a single default quenchant applied across all alloys.
When a part is hardened, default to tempering it per the application's target hardness/toughness balance before considering the process complete.
When verifying a heat-treated part's properties, default to actual hardness testing at representative or critical locations, never assuming a followed recipe guarantees spec compliance.
When heat-treating an asymmetric or mixed-thickness part, default to planning fixturing and orientation for distortion control before quenching, not addressing warping after it's already occurred.

Decision framework

Confirm the alloy, target case depth (if applicable), target hardness, and application requirements before setting process parameters.
Calculate time-temperature parameters for carburizing/hardening based on the verified diffusion/transformation relationship for this specific alloy and target, not a generic recipe.
Select quenchant, agitation, and fixturing appropriate to the alloy's hardenability and the part's geometry/distortion risk.
Quench per the calculated parameters, minimizing quench delay from furnace removal.
Temper per the target hardness/toughness balance for the application.
Verify actual hardness — and case depth, via cross-section testing on representative samples if applicable — against specification.
Document actual time-temperature parameters, quenchant/method used, and verification test results for the batch record.

Tools & methods

Furnace atmosphere control systems; quench tanks (oil, water, polymer) with agitation; hardness testers (Rockwell, Brinell); case depth cross-section/microhardness testing; fixturing and racking equipment for distortion control; tempering furnace scheduling. See references/playbook.md for a filled case-depth diffusion calculation and a quench-delay tracking worksheet.

Communication style

Heat treat batch records state actual time-temperature parameters, quenchant used, and hardness/case-depth test results, never "heat treated per procedure." Escalation about an out-of-spec part cites the specific measured hardness/case depth against target and the process step suspected, not "part didn't harden right."

Common failure modes

Assuming case depth scales linearly with time rather than calculating the actual diffusion-based relationship, producing case depth significantly different from target.
Using a default quenchant/agitation regardless of the specific alloy's hardenability requirement.
Treating hardening as the complete process and skipping or under-specifying tempering, leaving a part too brittle for service.
Releasing a part based on the recipe having been followed rather than actual hardness testing, missing a furnace atmosphere or quench-delay issue.
Having learned to distrust nominal recipe compliance, over-testing every part at multiple locations regardless of geometry or criticality, slowing throughput without a corresponding quality benefit on low-risk parts.

Worked example

A carburizing process targets a case depth of 1.0mm at a furnace temperature of 925°C. This furnace/alloy combination follows the verified relationship depth (mm) ≈ k × √(time in hours), with k = 0.5, confirmed from a prior batch that achieved 0.5mm case depth at 1 hour.

Naive read: Since 1 hour produced 0.5mm, and the new target is double that (1.0mm), simply double the time to 2 hours.

Expert reasoning: Case depth follows depth = k × √(time), not depth = k × time. Verifying k against the prior batch: 0.5 × √1 = 0.5mm, confirming k = 0.5 is correct for this furnace/alloy. Solving for the time needed to reach 1.0mm: 1.0 = 0.5 × √(time), so √(time) = 1.0 ÷ 0.5 = 2.0, meaning time = 2.0² = 4.0 hours — not the naively doubled 2 hours. If only 2 hours were used, the actual resulting depth would be 0.5 × √2 = 0.5 × 1.414 = 0.707mm — 29.3% short of the 1.0mm target (0.293 ÷ 1.0), despite doubling the time from the prior batch.

Deliverable — carburizing process parameter note:

> Target case depth: 1.0mm at 925°C, using this furnace/alloy's diffusion constant k=0.5 (depth = k × √time, verified from prior batch: 0.5mm at 1 hour matches 0.5 × √1 = 0.5mm). Naive linear-scaling approach (double time to 2 hours for double depth) would only achieve 0.5 × √2 = 0.707mm — 29.3% short of the 1.0mm target. Correct time: √time = 1.0 ÷ 0.5 = 2.0, time = 4.0 hours. Set carburizing time to 4.0 hours, not 2.0 hours, to hit the 1.0mm case depth target. Verify actual case depth via cross-section microhardness testing on a representative sample after processing.

Going deeper

references/playbook.md — filled case-depth diffusion calculation and a quench-delay tracking worksheet.
references/red-flags.md — signals with numeric thresholds for case depth, hardenability, and distortion problems.
references/vocabulary.md — terms of art generalists confuse or misuse.

Sources

ASM Handbook (Heat Treating volume) for carburizing diffusion behavior, quenchant selection by alloy hardenability, and tempering hardness/toughness relationships; general heat treat shop practice on quench delay control and distortion-control fixturing. Specific numeric examples (diffusion constants, case depths) in this file are illustrative and consistent with common heat treating practice — the specific furnace/alloy combination's empirically verified parameters always govern over the defaults here.

View SKILL.md source on GitHub · maturity: draft

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