SUS440B Stainless Steel: Medium-Carbon Martensitic Grade for Enhanced Wear Resistance

SUS440B belongs to the high-carbon martensitic family that dominates cutting tools, bearing components and wear parts. Within the JIS 440 series it is the balanced middle option: more edge-holding ability than SUS440A, noticeably more toughness than SUS440C, and a cost below SUS440C because it does not demand the same alloy control. This article gives the verified composition and property data, corrects several inaccuracies that circulate in supplier literature, and explains heat treatment and passivation so the grade performs as designed.

1. Composition and Equivalents

SUS440B is defined in JIS G4303 (bars) and JIS G4304/G4305 (plate, sheet and strip). Its US counterpart is 440B, UNS S44003, in ASTM A276 and A484; the Chinese designation is 8Cr17 in GB/T 1220.

Element (wt%)	SUS440B (JIS G4303)	440B (ASTM A276, UNS S44003)	8Cr17 (GB/T 1220)
Carbon	0.75–0.95	0.75–0.95	0.75–0.95
Silicon, max	1.00	1.00	1.00
Manganese, max	1.00	1.00	1.00
Phosphorus, max	0.040	0.040	0.035
Sulfur, max	0.030	0.030	0.030
Chromium	16.0–18.0	16.0–18.0	16.0–18.0
Molybdenum, max	-	0.75	-

Equivalent note: there is no direct EN 10088 designation for 440B. The closest European grades are X90CrMoV18 (1.4112) and X105CrMo17 (1.4125), both molybdenum/vanadium variants closer to 440C. When an EN substitution is requested, verify the actual composition delivered rather than assuming a one-to-one match.

2. Mechanical Properties and Hardness

JIS G4303 specifies the quenched-and-tempered condition for SUS440B with the following minimums: tensile strength ≥ 735 MPa and hardness ≥ 53 HRC. The table below summarises standard requirements and typical values.

Condition	Property	Value
Annealed (+A)	Hardness	≤ 255 HB (typical)
Quenched and tempered	Tensile strength	≥ 735 MPa (JIS requirement)
	Hardness	≥ 53 HRC (JIS requirement); 55–59 HRC typical after tempering at 180–220 °C
	Impact toughness	≈ 15–20 J (Charpy, typical for 180–220 °C temper)

Compared within the series: SUS440A reaches about 55–57 HRC, SUS440B 57–59 HRC and SUS440C 60–62 HRC after optimum tempering. Wear resistance increases with carbon and hardness; toughness decreases in the same direction. SUS440B is selected where the part must hold an edge or resist abrasion but still absorb moderate impact.

3. Heat Treatment Practice

Annealing: 850–900 °C, slow furnace cooling, for softness before machining (≤ 255 HB typical).

Hardening: austenitise at 1010–1070 °C and quench in oil. The 16–18 % chromium content requires full austenitising for carbide dissolution; hardening temperature and soak time must be controlled to avoid retained austenite.

Tempering: 150–200 °C for maximum hardness (55–59 HRC). Avoid the 300–450 °C range, where a sharp drop in impact toughness occurs. After tempering, cool in air promptly.

Dimensional change: about +0.1 % in quench-hardening, which must be allowed for in finished-ground components; stabilising by a sub-zero treatment reduces retained austenite for precision parts.

4. Corrosion Resistance and Passivation

As a martensitic grade, SUS440B offers only moderate corrosion resistance, suitable for dry or mildly humid environments. In service it relies on the same passive film as all stainless steels, but the chromium content available for passivation is partly bound in carbides, so the film is less robust than on 304. Practical rules:

Passivation per ASTM A967: use a nitric-acid (20–25 % at 50–60 °C, or 25–45 % at room temperature) or citric-acid (4–10 %) bath. The nitric/hydrofluoric mixtures used for pickling austenitic grades are unnecessary for 440B and involve hazardous chemistry - they are not recommended for a finishing step on hardened blades.

Surface protection: after passivation, apply a light anti-rust oil or wax for storage and humid service.

Service limit: continuous service is typically limited to −20 °C to 250 °C; above about 250–300 °C hardness falls off and tempering continues.

5. Typical Applications

Premium kitchen, hunting and tactical knives - edge retention with acceptable toughness

Bearing rings and balls for medium-speed, medium-load duty

High-pressure valve seats, discs and cores

Precision cutting tools and trimmer blades

Mould and die inserts for abrasive plastics

Marine and outdoor hardware where the part is protected by coating or oil film

Aerospace and instrument components with low impact demand

Frequently Asked Questions

What is the difference between SUS440A, SUS440B and SUS440C?

Carbon content: SUS440A 0.60–0.75 %, SUS440B 0.75–0.95 %, SUS440C 0.95–1.20 %. Higher carbon means higher attainable hardness and wear resistance (about 55–57, 57–59 and 60–62 HRC respectively) and lower toughness. SUS440B is the balanced middle choice.

Is SUS440B the same as EN 1.4109?

No. 1.4109 is X65CrMo14 (0.60–0.70 % C, 13.5–15.0 % Cr), a lower-carbon, lower-chromium grade. SUS440B has no direct EN 10088 equivalent; the closest European grades (X90CrMoV18, 1.4112) are Mo/V variants closer to 440C.

How do I avoid temper brittleness in SUS440B?

Never temper in the 300–450 °C range, where impact toughness collapses. Temper at 150–220 °C for maximum hardness and cool in air after tempering. For toughness-critical parts, raise the tempering temperature and accept lower hardness.

How should SUS440B be passivated?

Per ASTM A967, using a nitric-acid or citric-acid passivation bath. Avoid hydrofluoric-acid pickling mixtures on hardened components - they are hazardous and unnecessary for this grade. Rinse thoroughly, neutralise, and apply a protective oil film for storage.

Can SUS440B be used for bearing components?

Yes, for medium-speed and medium-load rings and balls. The high hardness (≥ 53 HRC, typically 55–59 HRC) and good dimensional stability support bearing accuracy; for heavier loads or higher speeds, SUS440C or a through-hardening bearing steel may be required.

Does SUS440B rust?

Yes, if exposed to water, chlorides or humid air without protection. It is corrosion resistant in dry environments only. For wet service, keep the surface passivated and oiled, or select a higher-chromium martensitic grade such as 440C (16–18 % Cr with free-chromium reserve) - and even then, protection is still required.