Heat Treatment and Temper Conditions for CNC Alloys
Temper conditions for CNC alloys: steel quench-and-temper hardness by temperature, 6061-O to T651, 7075-T73 and 17-4PH H900 to H1150 with NACE limits.
Carbon and alloy steel — heat-treatment processes and typical hardness
| Process | What it does | Typical hardness | Typical application | Governing standard |
|---|---|---|---|---|
| Annealing | Heat above or near the critical temperature, then cool very slowly in the furnace to soften and relieve stress | 160–217 HB (grade-dependent) | Machining stock, cold forming, stress relief before re-hardening | GB/T 16923; ASTM A29; SAE J412 |
| Normalizing | Heat 30–50 °C above Ac3/Acm, then cool in still air to refine grain and homogenize the structure | 187–300 HB (grade-dependent) | Large forgings, structural parts where quenching risks distortion | GB/T 16923; ASTM A29 |
| Quenching + tempering | Austenitize, quench to martensite in water or oil, then reheat below Ac1 to relieve stress and tune hardness | 24–57 HRC (depends on tempering temperature) | Finished engineering parts: shafts, gears, fasteners | GB/T 16924; ASTM A29 |
| Quench and high temper (Q&T) | Quench followed by high-temperature tempering at 500–650 °C, producing tempered sorbite with the best strength-toughness balance | 28–34 HRC (common supply band) | Connecting rods, bolts, gears, shafts under alternating load | GB/T 16924; GB/T 7232 |
What Hardness Should I Expect from Annealed Versus Normalized 1045 Steel?
Annealed 1045 sits at about 160–200 HB (163 HB typical hot-rolled), while normalized 1045 reaches about 187 HB with 640 MPa tensile — normalization trades some machinability for a stronger, more uniform structure.
| Condition | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness | Source |
|---|---|---|---|---|---|
| Hot-rolled / annealed | 565 (typical hot-rolled) | 310 | 16 | 163 HB; annealed band 160–200 HB | S21 / S22 / S5 |
| Normalized | 640 | 410 | 22 | 187 HB | S6 / S5 |
| Quenched and tempered | — | — | — | 220–290 HB | S5 |
| As-quenched (water, surface) | — | — | — | 55–60 HRC | S5 |
| Flame / induction hardened (surface) | — | — | — | 55–58 HRC, case depth 1.5–5 mm | S5 |
What Hardness Is 4140 Supplied at in the Pre-Hardened Condition?
Pre-hardened 4140 arrives at 28–34 HRC; the standard heat treatment tempered at 540 °C lands at 28–32 HRC and 1,000–1,100 MPa tensile. Chromium and molybdenum give 4140 far better hardenability than 1045, so it keeps its tempered hardness in larger sections — verify big bars on the mill test report, because hardness falls with section size.
| Condition / tempering temperature | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness | Source |
|---|---|---|---|---|---|
| Annealed | ~655 | ~415 | 25 | 197–217 HB | S4 |
| Normalized | ~1,020 | ~650 | 17 | 250–300 HB | S4 |
| As-quenched (oil) | — | — | — | 55–60 HRC | S4 |
| Q&T at 205 °C | ~2,000 | ~1,600 | 8 | 52–57 HRC | S4 |
| Q&T at 315 °C | ~1,700 | ~1,450 | 10 | 45–50 HRC | S4 |
| Q&T at 425 °C | ~1,400 | ~1,250 | 12 | 38–42 HRC | S4 |
| Q&T at 540 °C (standard HT) | 1,000–1,100 | 850–950 | 18 | 28–32 HRC | S4 |
| Q&T at 650 °C (high ductility) | ~850 | ~700 | 22 | 24–28 HRC | S4 |
| Pre-hardened as-supplied | — | — | — | 28–34 HRC | S4 |
How Hard Is Steel Immediately After Quenching, Before Tempering?
Both 1045 (water quench) and 4140 (oil quench) reach about 55–60 HRC as-quenched — but that untempered martensite is too brittle for service, so every quenched part is tempered back to its working hardness before use.
| Steel | Quench medium | As-quenched (HRC) | Tempered service hardness | Note |
|---|---|---|---|---|
| 1045 | Water | 55–60 | 220–290 HB (Q&T) | Shallow-hardening carbon steel; the core softens in large sections |
| 4140 | Oil | 55–60 | 28–32 HRC (tempered at 540 °C) | Cr-Mo alloy with deep hardening; pick it for bigger sections |
What Does Quenching and Tempering Mean and Why Is It Specified?
Quenching and tempering means quench to martensite, then reheat below the critical temperature; quench and high temper (500–650 °C) produces tempered sorbite — typically 28–32 HRC — the best strength-toughness balance for shafts and gears. Tempering always trades hardness for toughness: the same 4140 that leaves the quench at 55–60 HRC spans 52–57 HRC down to 24–28 HRC as the tempering temperature rises from 205 °C to 650 °C.
| Steel | Q&T supply band | After quench only | Why temper |
|---|---|---|---|
| 1045 | 220–290 HB | 55–60 HRC (water) | Untempered martensite cracks in service; tempering buys toughness back at a controlled hardness cost |
| 4140 | 28–34 HRC pre-hard | 55–60 HRC (oil) | High tempering at 500–650 °C forms tempered sorbite — the condition shafts and gears are specified in |
How Much Stronger Is 6061-T6 Compared with 6061-O?
6061-T6 reaches 310 MPa (45 ksi) tensile against 125 MPa (18 ksi) for 6061-O — about 2.5× — with yield 276 vs 55 MPa and hardness 95 vs 30 HB; heat treatment, not alloying, produces the whole difference. Standard minimums sit beside the typicals: 6061-T6 sheet and plate must reach 290 MPa (42 ksi) tensile and 240 MPa (35 ksi) yield, while 310/276 MPa are typical values.
| Alloy and temper | Tensile, MPa (ksi) | Yield, MPa (ksi) | Elongation (%) | Hardness (HB) | Source |
|---|---|---|---|---|---|
| 6061-O | 125 (18) | 55 (8) | 25–27 | 30 | S7 / S8 |
| 6061-T4 / T451 | 240 (35) | 145 (21) | 22 | 65 | S7 / S8 |
| 6061-T6 / T651 | 310 (45) | 276 (40) | 12–15 | 95 | S7 / S8 |
Is 6061-T651 Stronger Than 6061-T6?
No — both are 310 MPa (45 ksi) tensile and 276 MPa yield. T651 adds 1–3% permanent stretching after quenching to relieve quench stress, so it holds tighter dimensions during machining; that is why plate and panel work starts from T651 stock.
| Temper | Process | Mechanical difference | Governing standard | Source |
|---|---|---|---|---|
| O | Fully annealed — heated and slowly cooled to the softest state | Lowest strength, highest formability | ISO 2107; ASTM B209/B221 | S14 / S19 / S7 |
| T4 | Solution heat-treated, then naturally aged at room temperature | Moderate strength, still formable | ISO 2107; ASTM B209/B221 | S14 / S15 / S7 |
| T6 | Solution heat-treated, then artificially aged to peak strength | Highest strength for the alloy | ISO 2107; ASTM B209/B221 | S14 / S15 / S7 |
| T651 | T6 plus 1–3% stretching for stress relief before aging | Strength equal to T6; better dimensional stability in machining | ISO 2107; ASTM B209/B221 | S14 / S7 / S24 |
| T73 | Solution heat-treated, then overaged in a two-step aging treatment | Lower strength than T6; much better stress-corrosion resistance; mainly 7xxx series | AMS 4078; ASTM B209; GB/T 3880 | S9 / S2 |
What Is the T73 Temper and When Should I Choose It?
T73 is an overaged two-step temper chosen for stress-corrosion resistance in aerospace parts. 7075-T73 strength comes in two published bases kept as separate rows — the ASM Handbook typical 505 MPa and the ASTM B209 plate specification 460–475 MPa — and the two are never averaged. 6061 is not supplied in T73, which is why 7075 carries the numbers.
| Applicable basis | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness | Source |
|---|---|---|---|---|---|
| 7075-T73 — ASM Handbook typical (bar / forging reference) | 505 | 435 | 13 | — | S1 / S2 |
| 7075-T73 — ASTM B209 / AMS 4078 plate, by thickness (spec) | 460–475 | 385–390 | 5–8 | 126–140 HB | S9 |
| 7075-T6 — typical, for comparison | 572 (83 ksi) | 503 (73 ksi) | — | — | S7 |
What Is the Strongest Standard Aging Condition for 17-4PH?
H900, aged at 482 °C (900 °F) for 1 hour, gives a minimum 1,310 MPa (190 ksi) tensile, 1,170 MPa (170 ksi) yield and at least 40 HRC — the highest-strength, lowest-toughness end of the three standard conditions. All values are bar specification minimums per ASTM A564 / AMS 5643.
| Condition | Aging temperature / time | Tensile min, MPa (ksi) | Yield min, MPa (ksi) | Elongation min (%) | Hardness | Charpy V-notch (ft-lb) | Source |
|---|---|---|---|---|---|---|---|
| H900 | 482 °C (900 °F) / 1 h | 1,310 (190) | 1,170 (170) | 10 | ≥40 HRC (typical 44) | 10–15 (low) | S3 / S23 |
| H1025 | 552 °C (1,025 °F) / 4 h | 1,070 (155) | 1,000 (145) | 12 | ≥35 HRC (typical 38) | 35–45 (moderate) | S3 / S23 |
| H1150 | 621 °C (1,150 °F) / 4 h | 930 (135) | 725 (105) | 16 | 28–33 HRC (typical 33) | 60–80 (high) | S3 / S23 |
Which 17-4PH Condition Offers the Highest Toughness?
H1150, aged at 621 °C, keeps 930 MPa (135 ksi) minimum tensile with 16% elongation and 28–33 HRC — the highest Charpy toughness of the three. From H900 to H1150, tensile falls about 30% and yield about 38% while elongation rises from 10% to 16%: the higher aging temperature coarsens the copper-rich precipitates and trades strength for toughness. Charpy values are longitudinal representative bands and move with product form and orientation.
| Condition | Charpy V-notch (ft-lb) | Elongation (%) | Hardness | Best for |
|---|---|---|---|---|
| H900 | 10–15 (low) | 10 | ≥40 HRC | Highest-strength parts with no impact or cryogenic exposure |
| H1025 | 35–45 (moderate) | 12 | ≥35 HRC | The general-purpose balance condition |
| H1150 | 60–80 (high) | 16 | 28–33 HRC | Impact- and cryogenic-facing parts; also the NACE-compliant choice |
Can 17-4PH Be Used in NACE Sour-Gas Service?
Only H1150 — or H1025 with per-heat verification that actual hardness stays at or below 33 HRC — qualifies under NACE MR0175 / ISO 15156-3; H900 at a 40 HRC minimum does not qualify. The rule is hardness-driven: sour service caps hardness at 33 HRC.
| Condition | Spec hardness | NACE MR0175 status | Note |
|---|---|---|---|
| H900 | ≥40 HRC (typical 44) | Not qualified | Exceeds the 33 HRC sour-service cap by a wide margin |
| H1025 | ≥35 HRC (typical 38) | Conditional | Only with per-heat verification that actual hardness stays at or below 33 HRC |
| H1150 | 28–33 HRC (typical 33) | Compliant | The standard pick for sour service |
How to Use These Numbers
- Values marked min are specification minimums from the governing standard (ASTM / AMS); unmarked values are typical and move with product form, section size and furnace practice — design to the governing specification and the mill test report.
- Steel hardness falls with section size (the mass effect): 4140 pre-hard 28–34 HRC applies to the supplied bar size, so verify large sections on the mill test report rather than assuming the catalogue value.
- Representative heat-treatment parameters: 6061 solution treatment 530 °C water quench, T6 artificial aging 160 °C for about 18 h or 177 °C for about 8 h, annealing to O at 413 °C; 7075-T73 two-step aging 110 °C for 8 h plus 175 °C for 8 h, after solution treatment at 475 °C for 90 min and 3 days natural aging.
- 17-4PH conditions are one solution-treated parent (Condition A after cooling from about 1040 °C) aged at different temperatures — the H number is the aging temperature in °F. Bar uses ASTM A564/A564M, plate and sheet ASTM A693, aerospace parts AMS 5643/5604; the Chinese counterpart 05Cr17Ni4Cu4Nb sits in GB/T 1220.
- Quenched-but-untempered martensite (55–60 HRC in both anchor steels) is a machining and grinding hazard, not a delivery condition — order steel in the annealed, normalized or quenched-and-tempered state and let the heat treater do the rest.
Frequently Asked Questions
Annealed 1045 is about 160–200 HB (163 HB typical), while normalized 1045 reaches about 187 HB with 640 MPa tensile.
Pre-hardened 4140 is typically 28–34 HRC; standard HT tempered at 540 °C lands at 28–32 HRC and 1,000–1,100 MPa tensile.
Both 1045 (water quench) and 4140 (oil quench) reach about 55–60 HRC as-quenched, but untempered martensite is too brittle for service.
Quench plus high-temperature tempering at 500–650 °C to form tempered sorbite, typically 28–32 HRC — the best strength-toughness balance for shafts and gears.
6061-T6 reaches 310 MPa (45 ksi) tensile versus 125 MPa (18 ksi) for 6061-O — about 2.5× the strength.
No — both are 310 MPa (45 ksi) tensile and 276 MPa yield; T651 adds 1–3% stretching for stress relief, so it holds tighter dimensions in machining.
An overaged two-step temper: 7075-T73 gives 505 MPa tensile per ASM Handbook (460–475 MPa for plate per ASTM B209), chosen for stress-corrosion resistance.
H900, aged at 482 °C for 1 hour: minimum 1,310 MPa (190 ksi) tensile and at least 40 HRC — with the lowest toughness of the three.
H1150, aged at 621 °C: 930 MPa (135 ksi) tensile with 16% elongation and 28–33 HRC, the highest Charpy toughness of the three.
Only H1150, or heat-verified H1025 at maximum 33 HRC, qualifies under NACE MR0175; H900 at a 40 HRC minimum does not qualify.
Standards & Sources
- S1 — ASM Metals Handbook, Vol. 2 (Properties and Selection: Nonferrous Alloys), 10th edition: 7075-T73 typical values
- S2 — Peer-reviewed study of AA7075 forgings (PMC): T73 aging sequence and the ASM benchmark values
- S3 — JN Alloys, 17-4PH heat treatment: H900 / H1025 / H1150 condition data (cross-checked against ASTM A564 minimums)
- S4 — Nifty Alloys, 4140 steel heat treatment: tempering chart by temperature
- S5 — steelstandart grade data for 1045: annealed, Q&T and as-quenched hardness
- S6 — Published mill sheet, 1045 medium-tensile carbon steel bar, normalized properties
- S7 — Tech Steel, 6061 (UNS A96061) properties: temper-by-temper typicals and the 7075-T6 comparison
- S8 — Weltstahl, Aluminum 6061 data sheet: mechanical-property typicals
- S9 — 7075-T73 / T7351 sheet and plate data: mechanical properties by thickness (ASTM B209 basis)
- S14 — ISO 2107, aluminum temper designations (GB/T 16475 is the Chinese counterpart); S15 — AZoM aluminum classifications; S19 — GB/T 16475; S24 — aluminum temper guide (T651 stretching practice)
- S21 / S22 — 1045 annealed-condition data (The World Material; SAE 1045 mill data); S23 — Alloy 17-4PH quick-facts sheet: hardness and Charpy bands
- Governing standards carry the load-bearing minimums: ASTM A564/A564M, ASTM A693, ASTM B209/B221, AMS 4078/5643/5604, ISO 2107, SAE J412, GB/T 16923, GB/T 16924, GB/T 7232, GB/T 1220, NACE MR0175 / ISO 15156-3
- MatWeb was excluded (robots.txt blocks automated access); distributor data sheets carry only typical-value columns, while spec minimums cite the ASTM / AMS / GB originals
Related Pages
- /wiki/reference/hardness-conversion/ — Sibling quick table: convert between HB, HRC and HV hardness scales
- /wiki/reference/cutting-parameters/ — Sibling quick table: cutting speeds and feeds to run once the heat-treatment condition is fixed
- /wiki/reference/material-density/ — Sibling quick table: density for weight and material cost estimates