Steel A372 Grade H QT-C55
Heat-treated state
ASTM A372 Grade H
QT-C55 = class 55
Lowest-strength, highest-toughness class — pick where wall thickness is not weight-critical and maximum ductility/weldability is wanted.
Values for the QT-C55 condition
Other A372 Grade H tempers and conditions
A372 Grade H is listed in 3 conditions; see the grade overview at A372 Grade H.
Compare all 3 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
QT-C55This page Class 55 | Lowest-strength, highest-toughness class — pick where wall thickness is not weight-critical and maximum ductility/weldability is wanted. | 440 | 620 | 24 | 180 HB | — |
Class 65 | Balanced class — good compromise of strength, toughness and sour-service hardness limits for medium-pressure vessels. | 505 | 655 | 22 | 15 HRC | — |
QT-C70Default Class 70 | Most-specified class for thin-wall high-pressure cylinders and accumulators where wall thickness/weight is minimised. | 540 | 690 | 20 | 18 HRC | — |
What is A372 Grade H QT-C55?
A372 Grade H QT-C55 is A372 Grade H heat treated to QT-C55 — lowest-strength, highest-toughness class — pick where wall thickness is not weight-critical and maximum ductility/weldability is wanted. A372 Grade H QT-C55 has a yield strength of 440 MPa (63.8 ksi) and a tensile strength of 620 MPa (89.9 ksi) — stronger than 50% of steel grades. Elongation at break is 24% and the elastic modulus is 205 GPa (29.7 Msi). A372 Grade H QT-C55 has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,420°C (2,588 °F). QT-C70 is the default condition FabDigit quotes; QT-C55 is available on request or by drawing note.
Advantages
- Easy to machine — 62/100, better than 89% of steel grades
- Tough — resists crack growth — 120 MPa·√m (109.2 ksi·√in), better than 81% of steel grades
A372 Grade H QT-C55 properties
Typical room-temperature values for A372 Grade H QT-C55; 19 are specific to this condition. Other listed values are A372 Grade H reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
A372 Grade H QT-C55 has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,420°C (2,588 °F).
Mechanical17
A372 Grade H QT-C55 has a yield strength of 440 MPa (63.8 ksi) and a tensile strength of 620 MPa (89.9 ksi) — stronger than 50% of steel grades. Elongation at break is 24% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
A372 Grade H QT-C55 is rated for continuous service to 425°C (797 °F). It conducts heat at 42 W/m·K (24.3 BTU/hr·ft·°F), worse than 56% of steel grades. Thermal expansion is 12.3 µm/m·K (6.83 µin/in·°F).
Electrical3
A372 Grade H QT-C55 conducts electricity at 8 % IACS, better than 55% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (12/100), worse than 53% of steel grades.
Sustainability4
Producing a kilogram of A372 Grade H QT-C55 takes about 30 MJ of energy and emits 2.2 kg of CO₂ — more than 56% of steel grades. Typical recycled content is 35%.
Manufacturability7
A372 Grade H QT-C55's machinability is good (62/100, better than 89% of steel grades); weldability fair (50/100); formability fair (35/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with A372 Grade H QT-C55
Is A372 Grade H easy to machine?
Rough machine before final temper where possible; at 190–235 HB use carbide inserts, 100–160 m/min, rigid setups and generous coolant for deep bores.
Can A372 Grade H be welded?
Low-hydrogen process only, preheat 200–300 °C, interpass control, and PWHT below the tempering temperature; qualify procedures to the vessel code.
Can A372 Grade H be formed, bent or molded?
Hot forge 1100–1230 °C with finish above 900 °C, then austenitize, quench and temper the whole forging; cold forming after Q&T is impractical.
What surface finishes work on A372 Grade H?
Not anodizable — protect with shot blast + epoxy/zinc-rich paint, phosphate, or plating; nitriding can raise bore wear resistance if kept below the tempering temperature.
A372 Grade H chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| Cmedium-carbon Ni-Cr-Mo type per ASTM A372 Grade H; verify exact limits against current edition | 0.3 – 0.45 | 0.38 |
| Mn | 0.6 – 1 | 0.8 |
| Si | 0.15 – 0.35 | 0.25 |
| P | ≤ 0.03 | — |
| S | ≤ 0.03 | — |
| Nialloying for hardenability/toughness | 0.4 – 1 | 0.6 |
| Cr | 0.4 – 1.1 | 0.8 |
| Mo | 0.15 – 0.3 | 0.2 |
| Fe | balance | — |
A372 Grade H QT-C55 — frequently asked
What is A372 Grade H QT-C55 used for?
A372 Grade H QT-C55 is typically used for missile motor cases, high-pressure cylinders and artillery components. In short: highest A372 strength classes; missile case forging steel; machines better than Grade G.
What is the yield strength of A372 Grade H QT-C55?
A372 Grade H QT-C55 has a typical yield strength of 440 MPa (63.8 ksi) and a tensile strength of 620 MPa (89.9 ksi) — stronger than 50% of steel grades. Strength varies by condition: see the 3 listed tempers.
Is A372 Grade H QT-C55 easy to machine?
Reasonably — machinability is rated good (62/100, better than 89% of steel grades). Rough machine before final temper where possible; at 190–235 HB use carbide inserts, 100–160 m/min, rigid setups and generous coolant for deep bores.
Can A372 Grade H QT-C55 be welded?
With care — weldability is rated fair (50/100). Low-hydrogen process only, preheat 200–300 °C, interpass control, and PWHT below the tempering temperature; qualify procedures to the vessel code.
What surface finishes work on A372 Grade H QT-C55?
Not anodizable — protect with shot blast + epoxy/zinc-rich paint, phosphate, or plating; nitriding can raise bore wear resistance if kept below the tempering temperature.
Can A372 Grade H QT-C55 be formed, bent or molded?
Hot forge 1100–1230 °C with finish above 900 °C, then austenitize, quench and temper the whole forging; cold forming after Q&T is impractical.
What is the maximum service temperature of A372 Grade H QT-C55?
A372 Grade H QT-C55 is rated for continuous use to about 425°C (797 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between A372 Grade H QT-C70 and A372 Grade H QT-C55?
QT-C70 is the default condition — most-specified class for thin-wall high-pressure cylinders and accumulators where wall thickness/weight is minimised. QT-C55: lowest-strength, highest-toughness class — pick where wall thickness is not weight-critical and maximum ductility/weldability is wanted. Yield strength is 540 MPa in QT-C70 versus 440 MPa in QT-C55.
Sources
- ASTM A372/A372M — Standard Specification for Carbon and Alloy Steel Forgings for Thin-Walled Pressure Vessels — ASTM International (2020)
- ASTM A370 — Test Methods and Definitions for Mechanical Testing of Steel Products (hardness conversions) — ASTM International (2022)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels, and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol.19 — Fatigue and Fracture — ASM International (1996)
- NACE MR0175 / ISO 15156 — hardness limits for sour service — AMPP/ISO (2015)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.