440 stainless steel

Q1: What is 440 stainless steel, and how is it classified within stainless steel categories?

A1: 440 stainless steel is a family of martensitic stainless steels defined by the AISI standard, known for high carbon and chromium content. It is classified as a "hardenable" stainless steel due to its martensitic structure, which allows for significant strength and hardness improvements through heat treatment. Unlike austenitic grades (e.g., 304, 316), 440 is magnetic and prioritizes wear resistance and hardness over extreme corrosion resistance or ductility.

 

Q2: What are the main subgrades of 440 stainless steel, and what unifies their chemical composition?

A2: The 440 family includes three primary subgrades: 440A, 440B, and 440C (with 440F as a free-machining variant). Their unified chemical composition core (wt%) includes: Chromium (Cr) 16.00-18.00, Molybdenum (Mo) 0.75-1.25, Manganese (Mn) ≤1.00, Silicon (Si) ≤1.00, Phosphorus (P) ≤0.040, Sulfur (S) ≤0.030, and Nickel (Ni) ≤0.60. The key differentiator is carbon content: 440A (0.60-0.75), 440B (0.75-0.95), 440C (0.95-1.20), which directly drives hardness and wear resistance.

 

Q3: What are the general mechanical properties of 440 stainless steel (across subgrades) after heat treatment?

A3: When quenched (1010-1070°C, oil/air cooling) and tempered, all 440 subgrades exhibit high strength and hardness, with properties scaling with carbon content:

Tensile strength: 1550-1900 MPa (440A → 440C)

Yield strength: 1250-1550 MPa (440A → 440C)

Elongation: 5-8% (440C lowest, 440A highest)

Hardness (HRC): 54-60+ (440A: 54-58; 440B: 58-60; 440C: ≥60)

All subgrades offer excellent wear resistance and edge retention, with 440A providing the best toughness and 440C the highest hardness.

 

Q4: What are the common applications of 440 stainless steel (family-wide) across industries?

A4: 440 stainless steel is widely used in applications requiring a combination of wear resistance, moderate corrosion resistance, and high hardness, including:

Precision bearings and bearing components

Cutting tools (knives, blades, cutlery)

Surgical and medical instruments

Valve stems, seats, and pump shafts

Gears, pinions, and mechanical fasteners

Marine hardware and industrial machinery parts

Tooling and mold components for low-corrosion environments

 

Q5: What are the key processing considerations (heat treatment, welding, corrosion) for 440 stainless steel?

A5: - Heat Treatment: All 440 subgrades require quenching + tempering to achieve optimal properties. Avoid tempering between 475-550°C to prevent embrittlement. Lower tempering (150-300°C) maximizes hardness; higher tempering (400-600°C) improves toughness.