3Cr13 vs 4Cr13 – Composition, Heat Treatment, Properties, and Applications

Introduction

3Cr13 and 4Cr13 are martensitic stainless-steel grades that are widely used in components where moderate corrosion resistance must be balanced with wear resistance and strength (examples: cutlery, valves, shafts, and pump parts). Engineers, procurement managers, and manufacturing planners commonly face a tradeoff between mechanical strength/hardenability and ductility/weldability when selecting between these two grades.

The principal technical difference is the higher carbon content in 4Cr13 versus 3Cr13, which increases hardenability, attainable hardness, and strength at the expense of ductility and weldability. Because they share a similar chromium content, both offer comparable basic corrosion resistance relative to martensitic stainless steels, but their processing and final properties diverge primarily due to carbon and subtle alloying differences.

1. Standards and Designations

• Primary designation: Chinese national (GB) naming convention—3Cr13 and 4Cr13.
• Classification: Martensitic stainless steels (stainless, temperable, typically heat-treatable to martensite).
• Approximate family equivalents: These grades sit in the same general family as AISI/UNS martensitic stainless steels (commonly compared with the 410/420 series), but there is no guaranteed 1:1 correspondence across standards—consult specific standard documents or mill certificates for exact mappings.
• Other standards to consult for comparable stainless-martensitic materials: ASTM/ASME (A240 family for stainless plates/sheets; specific UNS numbers for bars), JIS (SUS martensitic series), and EN (martensitic stainless steel designations). Always verify composition and mechanical property tables in the applicable standard or supplier datasheet.

2. Chemical Composition and Alloying Strategy

Table: Typical chemical composition ranges (wt%). These are representative ranges frequently used in specifications; always verify the exact composition from material certificates.

Element	3Cr13 (typical range)	4Cr13 (typical range)
C	0.18 – 0.30	0.28 – 0.40
Mn	≤ 1.0	≤ 1.0
Si	≤ 1.0	≤ 1.0
P	≤ 0.04	≤ 0.04
S	≤ 0.03	≤ 0.03
Cr	12.0 – 14.0	12.0 – 14.0
Ni	≤ 0.6	≤ 0.6
Mo	≤ 0.1	≤ 0.1
V	≤ 0.1 (often not specified)	≤ 0.1
Nb	—	—
Ti	—	—
B	—	—
N	trace	trace

Notes: - The dominant deliberate alloying element is chromium (≈12–14%) to provide basic stainless behavior and support the martensitic matrix after quenching.
- The principal deliberate difference is carbon: 4Cr13 is formulated with higher carbon to raise hardenability and achievable hardness. Minor elements (Mn, Si) are mainly deoxidizers and will influence hardenability marginally; Mo, V (if present) will slightly enhance hardenability and tempering resistance. Ti/Nb/B are generally not present in notable amounts for these grades.

How alloying affects behavior: - Carbon: increases tensile strength, hardness, and wear resistance by promoting martensite and carbide formation; reduces ductility and weldability.
- Chromium: provides corrosion resistance (passive film) and increases hardenability; too low Cr reduces corrosion performance.
- Mo, V: when present in small amounts, raise tempering resistance and wear resistance.
- Mn/Si: influence deoxidation, strength, and toughness slightly.

3. Microstructure and Heat Treatment Response

Both grades are designed to be heat treated into a martensitic microstructure. Typical metallurgical routes and responses:

• As-supplied (annealed or normalized): ferritic/pearlitic with some carbides depending on carbon level. 3Cr13 will typically have a softer matrix with finer carbide distribution compared with 4Cr13 at the same processing state.
• Quenching and tempering: standard route to develop martensitic structure and desired hardness/toughness balance.
• Austenitize (typical range for similar martensitic stainless steels: 980–1050 °C) to dissolve carbides and form homogeneous austenite.
• Quench to transform austenite to martensite. Higher carbon (4Cr13) produces a higher proportion of hard martensite and retained carbides; 4Cr13 will typically achieve higher hardness for the same quench than 3Cr13.
• Temper at 150–650 °C depending on the target hardness/toughness tradeoff. Tempering reduces hardness but improves toughness; 4Cr13 requires more careful tempering to retain fatigue resistance and avoid excessive brittleness.
• Normalizing: can refine grain size and reduce segregation; followed by tempering as needed.
• Thermo-mechanical processing: cold work and subsequent tempering will influence dislocation density and final strength; 4Cr13 is more sensitive to hardening by cold work due to higher C.

Microstructural consequences: - 3Cr13: martensite with lower carbon content — somewhat lower hardness, better ductility and toughness when tempered comparably. - 4Cr13: martensite with higher carbon — higher hardness and wear resistance, higher risk of brittle martensite and carbide network if improperly heat treated.

4. Mechanical Properties

Table: Typical mechanical property ranges after typical quench & temper processing (note: values are illustrative; verify with supplier data).

Property	3Cr13 (typical)	4Cr13 (typical)
Tensile strength (MPa)	600 – 900	800 – 1100
Yield strength (0.2% offset, MPa)	350 – 650	550 – 900
Elongation (%)	10 – 20	6 – 15
Impact toughness (J, Charpy V-notch)	moderate (varies with temper)	lower (at same hardness)
Hardness (HRC, tempered)	HRC 38 – 52	HRC 45 – 58

Interpretation: - 4Cr13 can reach higher strength and hardness levels than 3Cr13 due to its higher carbon and slightly greater hardenability.
- 3Cr13 tends to be tougher and more ductile at equivalent tempering conditions; 4Cr13 trades ductility and toughness for more wear resistance and higher static strength.
- Impact toughness is highly dependent on tempering; for applications requiring resistance to shock or impact, proper tempering is critical and 3Cr13 typically offers a wider toughness window.

5. Weldability

Weldability is influenced primarily by carbon and hardenability. Higher carbon raises the risk of martensite formation in the heat-affected zone (HAZ), increasing the propensity for cracking and requiring preheat/Post Weld Heat Treatment (PWHT).

Useful predictive formulas (qualitative interpretation only): - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (weldability index): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative guidance: - Because 4Cr13 has higher C, its calculated $CE_{IIW}$ and $P_{cm}$ will typically be higher than 3Cr13, indicating poorer weldability and a greater likelihood of HAZ hardening and cold cracking.
- Best practice: control preheat, limit interpass cooling rates, use appropriate filler metals (matching or slightly lower carbon), and apply PWHT where required to temper HAZ martensite. 3Cr13 is more tolerant of conventional welding practices but still may require preheat for thicker sections or restraint conditions.

6. Corrosion and Surface Protection

• Both grades are stainless martensitic (≈12–14% Cr): they form a protective passive layer and have better corrosion resistance than plain carbon steels but are inferior to austenitic (304/316) and duplex grades in aggressive media.
• PREN (Pitting Resistance Equivalent Number) is generally not useful for these low-Mo, low-N martensitic stainless steels. For completeness: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• With Mo and N typically near zero in these grades, PREN values are low compared with duplex or austenitic alloys; therefore, these grades are suitable for mildly corrosive environments (atmospheric, mildly acidic/alkaline, limited chloride exposure) but not for severe chloride-containing media without coatings or cathodic protection.
• Surface protection for non-stainless comparables is not applicable; for these stainless martensitics, common protective measures include passivation after fabrication, electroplating, controlled polishing, and in-service protective coatings (organic paints, sacrificial coatings) when chloride or pitting risk is significant.

7. Fabrication, Machinability, and Formability

• Machinability: Higher-carbon 4Cr13 tends to be harder in the annealed condition and will cause greater tool wear; however, in the annealed condition both grades machine reasonably well with proper tooling and speeds. Hardened 4Cr13 will be harder to machine if not softened.
• Formability: 3Cr13 offers better cold forming and bendability than 4Cr13 due to lower carbon; deep drawing or severe forming is limited for both compared with austenitic stainless steels.
• Grinding, polishing, and surface finishing: 4Cr13's higher hardness gives better wear resistance in service but may need more aggressive finishing operations. Heat treatment and tempering before final machining/finishing is recommended to avoid distortion.
• Heat treatment distortion: Both grades are prone to distortion during quench and temper operations; careful fixturing, gradual cooling, and appropriate machining allowances are required.

8. Typical Applications

3Cr13 – Typical Uses	4Cr13 – Typical Uses
Knife blades and cutlery where balanced toughness and corrosion resistance are needed	Cutting tools and knives where higher edge retention and wear resistance are desired
Pump shafts, valve components with moderate wear demands	Wear-prone components, rollers, pins, and parts requiring higher hardness
Automotive trim, fasteners, and fittings where some bending/forming is required	Small-volume bearing components, wear pins, and hardened shafts
General-purpose martensitic stainless parts where welding/repairability is a factor	Parts where through-hardening and higher static strength are primary requirements

Selection rationale: - Choose 4Cr13 where edge retention, higher hardness and wear resistance are primary; choose 3Cr13 where ductility, impact resistance, and easier fabrication/welding are important. Cost considerations and surface finishing requirements also influence the decision.

9. Cost and Availability

• Cost: 4Cr13 is typically priced slightly higher than 3Cr13 due to the higher carbon content and the processing required to achieve and control higher hardness properties; however, price differences are modest compared with higher alloy grades (e.g., Mo-bearing martensitics or austenitics).
• Availability: Both grades are widely available in regions with established stainless-steel supply chains (sheets, bars, strips, blanks). Product form (bars, plate, strip) and finishing (cold-rolled, annealed, hardened) will influence lead times and cost. For large-volume procurement, verify mill certificates and batch testing for carbon content to ensure the intended mechanical properties.

10. Summary and Recommendation

Table: Quick comparative summary

Attribute	3Cr13	4Cr13
Weldability	Better (lower carbon)	Lower (higher carbon, more HAZ risk)
Strength–Toughness balance	Moderate strength with better toughness	Higher strength and hardness, lower toughness
Cost	Slightly lower	Slightly higher

Conclusion and practical recommendation: - Choose 3Cr13 if you need a balanced martensitic stainless with better ductility and weldability, easier formability, and slightly lower cost—appropriate for components that require some impact resistance, repairability, or moderate wear resistance. - Choose 4Cr13 if the design prioritizes higher hardness, wear resistance, and static strength where edge retention or abrasive wear are critical and where tighter heat-treatment control is acceptable; expect greater attention to weld procedures, preheat, and tempering to avoid brittleness.

Final note: The exact selection should be validated against supplier mill certificates, component geometry, restraint conditions during welding, and the specific service environment (corrosive media, temperature, cyclic loading). For critical applications, request material test reports (composition, hardness, tensile, and impact data) and perform qualification testing (weld trials, heat-treatment trials) before serial production.