The 400 series stainless steel strip represents a family of
ferritic and martensitic stainless steels renowned for their
durability, magnetic properties, and cost-effectiveness.
Distinguished by their chromium-rich composition (typically 11-30%
chromium) and minimal to no nickel content, these strips are widely
favored in industries where corrosion resistance, strength, and
affordability intersect. Below is a detailed exploration of their
characteristics, manufacturing, and diverse uses.
Core Composition and Metallurgy
Unlike austenitic stainless steels (such as 304 or 316), 400 series
strips are primarily ferritic or martensitic, defined by their
crystalline structure. Ferritic grades (e.g., 430) have a
body-centered cubic (BCC) structure, while martensitic grades
(e.g., 410) feature a tetragonal structure formed through heat
treatment. Both types rely on chromium for corrosion
resistance—chromium reacts with oxygen to form a passive chromium
oxide layer, shielding the underlying metal from rust and chemical
attack.
Notably, 400 series strips contain little to no nickel, a key
differentiator from austenitic grades. This absence reduces
material costs significantly, making them a budget-friendly
alternative in applications where extreme corrosion resistance
(e.g., in saltwater or acidic environments) is not critical.
Molybdenum is sometimes added to enhance corrosion resistance in
specific grades (e.g., 434), while carbon is included in
martensitic types to enable hardening via heat treatment.
Key Properties
400 series stainless steel strips exhibit a unique set of
properties that make them suitable for targeted applications:
- Magnetism: All 400 series strips are magnetic due to their ferritic
or martensitic structure, a trait valuable in electronics,
automotive sensors, and magnetic components.
- Corrosion Resistance: Moderate to good, depending on the grade.
Ferritic grades like 430 offer better resistance to atmospheric and
mild chemical exposure than martensitic types, which are more prone
to rust in humid or salty conditions.
- Heat Resistance: Ferritic grades retain strength at elevated
temperatures (up to 800°C) without scaling, making them ideal for
high-heat environments like furnace parts or exhaust systems.
- Mechanical Strength: Martensitic grades (e.g., 410, 420) achieve
high hardness and tensile strength (up to 1,200 MPa) after heat
treatment, surpassing many ferritic and even some austenitic
steels.
- Formability: Ferritic strips (e.g., 430) are easily cold-rolled,
bent, or stamped into complex shapes, though they are less ductile
than austenitic grades. Martensitic types are more brittle when
hardened but can be machined effectively in their annealed state.
Common Grades and Their Uses
The 400 series encompasses several grades, each tailored to
specific needs:
- 410: A martensitic grade with 11.5-13.5% chromium. It is
heat-treatable, offering high strength and wear resistance.
Applications include cutlery, valves, pump shafts, and surgical
instruments where sharpness and durability are critical.
- 420: A higher-carbon martensitic grade (12-14% chromium) that
achieves exceptional hardness (up to 50 HRC) after quenching and
tempering. Used in blades, precision tools, and industrial knives.
- 430: The most widely used ferritic grade, containing 16-18%
chromium. It balances corrosion resistance, formability, and cost,
making it suitable for automotive trim, kitchen appliances (e.g.,
refrigerator doors), and decorative panels.
- 434: A ferritic grade with added molybdenum (0.75-1.25%), enhancing
corrosion resistance. Used in automotive exhaust systems and
outdoor fixtures exposed to moisture.
- 444: A high-chromium (17-20%) ferritic grade with molybdenum and
niobium, offering superior corrosion resistance to 430. Ideal for
water heaters, chemical storage tanks, and marine components.
Manufacturing Process
400 series stainless steel strips undergo a precision manufacturing
process to ensure consistency:
- Melting and Casting: Chromium, iron, and other alloys are melted in
electric arc furnaces, then cast into slabs.
- Hot Rolling: Slabs are heated and rolled into thick coils to refine
grain structure and reduce thickness.
- Cold Rolling: Coils are cold-rolled to achieve precise thickness
(0.01-3mm) and surface finishes (e.g., 2B matte or polished).
- Annealing: Ferritic grades are annealed at 700-900°C to relieve
stress and restore ductility. Martensitic grades undergo heat
treatment (quenching and tempering) to harden.
- Slitting: Coils are slit into narrow strips of exact widths using
automated machines, ensuring tight tolerances for industrial use.
Advantages and Limitations
Advantages:
- Cost-effectiveness: Lower nickel content reduces material costs
compared to austenitic grades.
- Magnetic properties: Essential for applications requiring magnetic
responsiveness.
- Heat and wear resistance: Martensitic grades excel in high-stress,
high-friction environments.
- Recyclability: Like all stainless steels, 400 series strips are
100% recyclable, supporting sustainability.
Limitations:
- Reduced corrosion resistance: Less suitable for aggressive
environments (e.g., seawater, strong acids) compared to 316
austenitic steel.
- Lower ductility: Martensitic grades are prone to brittleness when
hardened, limiting complex forming.
