Alloy Steel: Properties and Uses

Introduction to Alloy Steels

• Alloy steels are a type of steel that contains other elements besides iron and carbon.
• The addition of other elements improves the properties of steel, making it suitable for various applications.

Classification of Alloy Steels

• Low alloy steels:
  • Contain up to 3-4% of one or more alloying elements.
  • Similar microstructure to plain carbon steels.
  • Used for increasing strength, toughness, and hardenability.
• High alloy steels:
  • Possess structures and require heat treatments different from plain carbon steels.
  • Examples: high-speed tool steels, stainless steels, and maraging steels.

High-Speed Tool Steels

• Composition: Tungsten, chromium, vanadium, and carbon.
• Properties:
  • High hardness and wear resistance.
  • Retain hardness at high temperatures.
• Applications:
  • Cutting tools: drills, taps, end mills, and saw blades.

Stainless Steels

• Composition: Chromium (min. 12%) and sometimes nickel.
• Properties:
  • Corrosion resistance due to chromium oxide film.
  • High-temperature resistance.
• Applications:
  • Kitchen utensils, medical equipment, and architectural features.

Maraging Steels

• Composition: Nickel, cobalt, and small amounts of titanium and other elements.
• Properties:
  • High strength and toughness.
  • Low carbon content.
• Applications:
  • Aerospace and defense industries.

Effects of Alloying Elements

• Carbide formers: Cr, W, Ti, V, and Mo.
• Austenite stabilizers: Ni, C, and N.
• Ferrite stabilizers: Cr, Mo, Si, Ti, and V.
• Grain refiners: Al, Ti, and Nb.

Specific Alloying Elements

• Carbon (C): increases hardness and strength.
• Chromium (Cr): improves corrosion resistance and hardness.
• Nickel (Ni): improves toughness and corrosion resistance.
• Manganese (Mn): improves strength and deoxidizes the steel.
• Molybdenum (Mo): enhances strength, hardness, and corrosion resistance.
• Vanadium (V): improves strength, toughness, and wear resistance.

Heat Treatment

• Austenitizing: heating to form austenite structure.
• Quenching: rapid cooling to achieve desired properties.
• Tempering: heating to reduce brittleness and improve toughness.

Specific Steel Types

• OHNS (O1) steel: high-speed tool steel with high hardness, toughness, and wear resistance.
• Hadfield's manganese steel: high-manganese steel with high toughness and wear resistance.
• High-speed steel (HSS): used for cutting tools due to high hardness and wear resistance.### Ferritic Stainless Steels
• Magnetic
• Good corrosion resistance, particularly in less severe environments
• Good ductility and formability, but less than austenitic grades
• Lower cost due to lower nickel content
• Limited hardenability, only by cold working
• Applications: automotive parts, kitchen equipment, industrial equipment, and architecture

Austenitic Stainless Steels

• Composition: 16-26% Cr, 6-22% Ni, low C (<0.08%), and other elements like Mn, N, and Mo
• Non-magnetic, but can become slightly magnetic when cold worked
• Excellent corrosion resistance in a wide range of environments
• Good weldability without requiring post-weld heat treatment
• Good toughness and ductility at high and low temperatures
• Hardenable by cold working
• Applications: food and beverage equipment, medical devices, chemical and petrochemical industry, construction, and household items

Martensitic Stainless Steels

• Composition: 11.5-18% Cr, higher C (0.1-1.2%), and sometimes Ni for toughness
• Magnetic
• High strength and hardness, can be significantly hardened by heat treatment
• Moderate corrosion resistance, lower than austenitic and ferritic grades
• Good wear resistance due to high hardness
• Heat treatable to achieve a wide range of hardness and strength levels
• Applications: cutlery and blades, turbine blades and engine parts, valves and pumps, and springs and fasteners

Weld Decay

• Type of intergranular corrosion that occurs in some stainless steels, particularly austenitic grades
• Occurs in the heat-affected zone (HAZ) adjacent to the weld
• Caused by chromium carbide precipitation and chromium depletion
• Prevention methods: using low-carbon grades, stabilized grades, heat treatment, controlled welding techniques, and proper filler materials
• Detection methods: sensitization tests (e.g., Strauss test) and microstructural analysis
• Consequences: inter-granular corrosion, reduced mechanical properties, and premature failure