SS420 Stainless Steel: Complete Guide for Plastic Moulds, Cutlery, Shafts & High-Hardness Applications
By Starship Alloys | Forged & Rolled Stainless Steel, Tool Steel and Special Alloys
When engineers need a stainless steel that can be hardened to 50 HRC or higher, resists wear like a tool steel, and still offers meaningful corrosion resistance, one grade consistently rises to the top: SS420.
Known as "surgical steel" in medical contexts, "cutlery steel" in the knife industry, and simply the standard plastic mould steel across Indian tool rooms, SS420 sits in a unique position. It is the hardest of the 12% chromium martensitic stainless steels — significantly harder than its cousin SS410 — while maintaining respectable corrosion resistance in mild environments.
At Starship Alloys, we supply forged SS420 round bars and flat bars. In this comprehensive guide, we will cover everything a tool room, procurement engineer, or manufacturer needs to know about this versatile high-hardness stainless grade.
What is SS420 Stainless Steel?
SS420 is a martensitic stainless steel containing approximately 12-14% chromium and a deliberately elevated carbon content of 0.15% minimum (typically 0.20-0.40%). The higher carbon content is the fundamental difference between SS420 and SS410 — and it is what unlocks SS420's ability to reach hardnesses that put it in league with true tool steels.
Unlike austenitic grades (SS304, SS316), SS420 can be hardened by heat treatment through the martensitic transformation. The higher carbon content means more carbon is available to form the hard martensitic structure and, after tempering, hard chromium carbides that resist wear.
SS420 is magnetic in all conditions and cannot be softened by any treatment other than annealing. It is the natural choice when your application demands stainless corrosion resistance combined with the highest hardness practically achievable in a plain-chromium stainless steel.
SS420 Equivalent Grades — International Cross Reference
SS420 is specified under numerous designations across international standards:
AISI / SAE: 420
UNS: S42000
DIN / EN: X20Cr13
Werkstoff Nr.: 1.4021
JIS (Japanese): SUS420J1 (0.15-0.25%C) / SUS420J2 (0.26-0.40%C)
BS (British): 420S37 / 420S45
AFNOR (French): Z20C13
GOST (Russian): 20X13
Swedish SS: 2303
Chinese GB: 2Cr13 / 3Cr13
Variants within the SS420 family:
There are several SS420-based grades with slightly different chemistries for specific applications:
SS420 (standard): 0.15% carbon minimum. General-purpose grade.
SS420A / 420J1: Lower carbon (0.15-0.25%), better toughness, moderate hardness (48-50 HRC).
SS420B / 420J2: Medium carbon (0.26-0.40%), higher hardness (50-55 HRC), the most common industrial grade.
SS420C: Higher carbon (0.35-0.45%), maximum hardness (55-58 HRC), often used for cutlery and bearings.
SS420F: Free-machining variant with sulphur addition (similar concept to SS416).
Always verify the exact carbon range specified in your Technical Delivery Conditions, as different variants can yield significantly different hardness after heat treatment.
SS420 Chemical Composition
The typical chemical composition of SS420 as per ASTM A276 / ASTM A479 is:
Carbon (C): 0.15% min (typically 0.20 – 0.40% depending on variant)
Chromium (Cr): 12.00 – 14.00%
Manganese (Mn): 1.00% max
Silicon (Si): 1.00% max
Phosphorus (P): 0.040% max
Sulphur (S): 0.030% max
Nickel (Ni): 0.75% max
Iron (Fe): Balance
The critical role of carbon:
The 0.15% minimum carbon in SS420 — compared to the 0.08-0.15% range in SS410 — is what fundamentally distinguishes these two grades. Higher carbon means:
More martensite formation during quenching, and therefore higher achievable hardness
More chromium carbide precipitation during tempering, contributing to wear resistance
Higher tensile strength and yield strength at all conditions
Reduced weldability — high carbon promotes hot cracking in the weld zone
Reduced toughness at maximum hardness — the harder martensite is inherently more brittle
Slightly reduced corrosion resistance in the tempered condition, because carbon locks up chromium in carbides
SS420 Mechanical Properties
The mechanical properties of SS420 vary dramatically depending on the heat treatment condition and the specific carbon variant. Approximate values for standard SS420 (0.20-0.30% C):
Annealed Condition:
Tensile Strength: ~590 MPa (86,000 psi)
Yield Strength: ~340 MPa (49,000 psi)
Elongation: ~15-20%
Hardness: ~195-235 HB (92-98 HRB)
Hardened & Tempered Condition (typical for industrial use):
Tensile Strength: 1550-1930 MPa (225-280 ksi)
Yield Strength: 1200-1500 MPa
Elongation: 8-15%
Hardness: 48-56 HRC (working range 48-52 HRC for most applications)
Charpy V-notch impact: 15-25 J (varies with hardness)
Physical Properties:
Density: 7.74 g/cm³ (0.280 lb/in³)
Thermal Conductivity: 24.9 W/m·K at 100°C
Coefficient of Thermal Expansion: 10.3 × 10⁻⁶/°C (20-100°C)
Modulus of Elasticity: 200 GPa
Melting Point: 1454-1510°C
Magnetic: Yes — martensitic structure, magnetic in all conditions
Heat Treatment of SS420 — The Key to Its Performance
Correct heat treatment is what transforms an ordinary-looking SS420 bar into a component that can hold an edge, resist wear, and last for years. Getting it wrong wastes the material's potential entirely.
Annealing
Anneal for maximum softness before machining.
Heat slowly to 840-900°C (1550-1650°F) and hold until uniformly heated. Furnace cool at a controlled rate (approximately 20°C per hour) down to 600°C, then air cool to room temperature.
Expected hardness after full annealing: 195-235 HB. This is the softest condition and the recommended state for all machining operations.
Process (Sub-Critical) Annealing
Heat to 735-785°C (1355-1445°F) and air cool. Faster and cheaper than full annealing, this provides acceptable softness for less demanding machining.
Hardening
Preheating is important for SS420 to avoid thermal shock and cracking. Preheat to 760-790°C (1400-1450°F), hold until uniform. For heavy sections, use a two-stage preheat: 540°C first, then 790°C.
Austenitising: Heat to 980-1065°C (1800-1950°F). The most common austenitising temperature is 1010-1030°C. Hold for 15-30 minutes depending on section size.
Quenching: Oil quench is preferred for sections thicker than 6mm. Air cooling is acceptable for thin sections. Water quenching should be avoided due to the high risk of cracking with SS420's high carbon content.
Sub-zero treatment (optional but recommended for critical applications): After quenching to room temperature, immediate sub-zero treatment at -75°C (-100°F) for 1-2 hours transforms any retained austenite to martensite. This is particularly valuable for precision components that must maintain dimensional stability in service.
Critical: Temper immediately after quenching. SS420 in the as-quenched condition is extremely brittle and can crack spontaneously from residual stresses if left untempered overnight.
Tempering
Tempering temperature directly controls the final hardness and mechanical properties. SS420 is unusual in that the tempering range is deliberately restricted to preserve corrosion resistance.
Tempering temperature vs. hardness (approximate):
150-200°C → 54-56 HRC (maximum hardness, cutlery, ball bearings)
250-300°C → 52-54 HRC (surgical instruments, knife blades)
350°C → 48-50 HRC (plastic moulds, general industrial)
400°C → 45-48 HRC (shafts, wear parts)
Critical Warning — The 425-600°C Temper Range:
Do NOT temper SS420 between 425°C and 600°C for any application requiring corrosion resistance or impact toughness. This range causes two serious problems:
Loss of corrosion resistance: Chromium carbides precipitate along grain boundaries, depleting the surrounding matrix of chromium and creating regions vulnerable to corrosion.
Temper embrittlement: Impact toughness drops dramatically without a proportional decrease in hardness — you get a component that is both brittle and less corrosion resistant.
For maximum corrosion resistance, temper at 300°C or below. For applications requiring higher toughness at the expense of some hardness and corrosion resistance, temper above 600°C.
SS420 vs SS410: The Higher-Hardness Cousin
SS420 vs SS440C: When Even More Hardness is Needed
Applications of SS420 in Indian Industry
SS420 finds extensive use across sectors requiring the combination of high hardness and stainless corrosion resistance:
Plastic Injection Moulds
This is the single largest application for SS420 in India. Plastic mould cavities, cores, ejector components, and inserts are commonly manufactured from SS420. The material offers:
Hardness (48-52 HRC) that resists the abrasion of filled plastics
Excellent polishability for mirror-finish mould surfaces
Corrosion resistance against acidic or moisture-containing plastic resins
Dimensional stability throughout the moulding lifecycle
For mould cavities requiring optical-grade polish, ESR-quality SS420 or its modified variants (like DIN 1.2083) are preferred.
Cutlery and Kitchen Knives
The Indian cutlery industry uses SS420 extensively for kitchen knives, kitchenware, and food service equipment. The steel provides:
Hardness (54-56 HRC) that holds a cutting edge
Corrosion resistance against food acids
Easy sharpening
Cost-effectiveness compared to premium knife steels
Higher carbon variants (SS420C) are preferred for premium kitchen knives requiring superior edge retention.
Surgical and Dental Instruments
Widely known as "surgical stainless steel," SS420 is used for surgical scalpels, dental instruments, scissors, forceps, and various medical tools. Key properties:
Autoclave sterilization compatibility
Corrosion resistance to disinfectants and biological fluids
Hardness for edge retention on cutting instruments
Excellent polishability for smooth, cleanable surfaces
Bearings and Precision Components
Small ball bearings, roller bearings, and precision wear components where standard bearing steels (52100) would suffer from corrosion. SS420 in hardened condition (54-56 HRC) provides adequate load capacity with meaningful corrosion protection.
Pump and Valve Components
Valve seats, valve balls, plunger rods, and pump internals where hardness for wear resistance is combined with mild corrosion environments. SS420 is particularly common in fluid handling equipment for chemical, pharmaceutical, and food industries.
Gauges, Measuring Tools, and Precision Instruments
Snap gauges, plug gauges, thickness gauges, calibration blocks, and precision measurement tools. SS420's dimensional stability after proper heat treatment and sub-zero conditioning makes it excellent for gauging applications.
Shafts and Spindles
Where SS410 does not provide adequate hardness for wear resistance at bearing surfaces, SS420 offers a step-up in hardness capability while maintaining reasonable stainless properties.
Forging SS420 — Process at Starship Alloys
SS420 requires careful forging due to its high carbon content:
Forging Temperature Range: 1095-1204°C (2000-2200°F)
Heat slowly and uniformly to forging temperature. The high carbon content makes SS420 sensitive to thermal shock — rapid heating can cause internal cracking. Do not forge below 900°C (1650°F).
Post-Forging Cooling:
SS420 must be cooled slowly after forging. Rapid cooling from forging temperature will produce hard, brittle martensitic structure with severe internal stresses that lead to cracking. Standard practice is either furnace cooling from 843°C, or burying the forging in warm dry lime or ashes.
Post-Forging Annealing:
A full annealing cycle is essential after forging. Direct machining of as-forged SS420 is not practical due to the extreme hardness and brittleness of the as-cooled structure.
At Starship Alloys, our SS420 process follows a strict sequence:
Melting: EIF melting with LRF treatment for optimal chemistry control. VD (Vacuum Degassing) available for enhanced cleanliness on request.
Ingot Cooling: Slow controlled cooling of ingots to prevent internal cracking.
Forging: Press or hammer forging with 3.5-4.5× reduction ratio for thorough microstructural refinement.
Full Annealing: Controlled annealing cycle to achieve maximum softness and homogeneity.
NABL-Approved Testing: Chemistry, mechanical properties, hardness, ultrasonic examination.
Proof Machining: Final machining to customer-specified dimensions and tolerances.
SS420 Available Forms at Starship Alloys
We supply SS420 in the following forged forms:
Forged Round Bars: Custom sizes available on request Forged Flat Bars: Custom sizes available on request
Supply Conditions:
Annealed (195-235 HB) — standard supply, optimal for machining
Hardened and tempered to customer-specified hardness ranges
Sub-zero treated for maximum dimensional stability
All material ships with complete NABL-approved test certificates covering chemistry, mechanical properties, hardness, and ultrasonic examination.
Frequently Asked Questions About SS420
Is SS420 magnetic? Yes. SS420 is martensitic and magnetic in all conditions — annealed, hardened, or tempered.
Can SS420 be welded? SS420 has limited weldability. Welding is possible but requires preheating to 200-320°C, use of matching or austenitic filler (E309), interpass temperature control, and post-weld annealing at 675-750°C. For applications requiring welding, SS410 or SS304L is usually a better choice.
What is the maximum hardness of SS420? Standard SS420 (0.20-0.30% C) reaches 52-54 HRC after typical heat treatment. Higher-carbon variant SS420C can reach 56-58 HRC. As-quenched hardness can approach 58 HRC but tempering is essential to reduce brittleness.
Is SS420 the same as "surgical stainless steel"? "Surgical stainless steel" is a general marketing term rather than a strict metallurgical designation. However, SS420 is one of the most common grades sold as surgical stainless, particularly for cutting instruments. SS304L and SS316L are also often called surgical grades for non-cutting instruments.
What is the difference between SS420 and 2Cr13/3Cr13? 2Cr13 and 3Cr13 are Chinese GB standard designations that correspond to SS420 variants. 2Cr13 is roughly equivalent to lower-carbon SS420 (~0.20% C), while 3Cr13 is equivalent to higher-carbon SS420 (~0.30% C). All are 12-14% chromium martensitic stainless steels.
What is the maximum service temperature for SS420? SS420 maintains its properties up to approximately 427°C in continuous service. Above 650°C, scaling becomes significant. For extended service above 427°C, the material begins to soften and lose corrosion resistance permanently — not recommended.
Can SS420 be used for cryogenic applications? No. Martensitic stainless steels including SS420 exhibit ductile-to-brittle transition at low temperatures. For cryogenic service, austenitic grades (SS304L, SS316L) are appropriate.
What is the density of SS420? The density of SS420 is approximately 7.74 g/cm³ (0.280 lb/in³).
Is SS420 food-safe? Yes, SS420 is used in food processing equipment, cutlery, and food service applications. Its corrosion resistance is adequate for contact with most food products, though for aggressive acidic environments, SS316L may be preferred.
Why Source SS420 from Starship Alloys?
High-Carbon Steel Expertise: SS420 is significantly more sensitive to melting practice, forging temperature control, and cooling rate than lower-carbon grades. Our process discipline — controlled slow cooling of ingots, adequate forging reduction, and precise annealing — ensures the material you receive is free of internal defects.
Consistent Carbon Content: The carbon range in SS420 is critical because it directly determines achievable hardness. Our melt chemistry is controlled tightly within customer-specified ranges, with every batch verified by NABL-approved chemical analysis.
Custom Heat Treatment: Whether you need annealed SS420 for machining, or pre-hardened and tempered material at a specific hardness for direct use, we manufacture to your Technical Delivery Conditions.
Technical Guidance: Not sure whether standard SS420, higher-carbon SS420C, or ESR-quality material is right for your application? Our team understands the trade-offs and provides informed recommendations.
Get a Quote for SS420 Forged Bars
Whether you need forged round bars or flat bars in SS420 for plastic moulds, cutlery manufacturing, surgical instruments, valve components, or any high-hardness stainless application, we are ready to discuss your requirements.
Phone: +91 9163 15 0171 | +91 9831740471 Email: mail@starshipalloys.com WhatsApp: +91 9163 15 0171 Visit: www.starshipalloys.com/ss420
Head Office: 190, Girish Ghosh Road, Belurmath, Howrah, West Bengal – 711202, India
Starship Alloys is a Howrah-based manufacturer and supplier of forged and rolled stainless steel, tool & die steel, and special alloys. We serve clients across India with custom-manufactured steel products backed by NABL-approved testing.
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