Properties and Applications of Fe-C Alloys and Stainless Steels

Fe-C Alloys: Alloy Elements

• Mn <1.6%: Descaling and desulfurization. Increases tensile strength (Re), ultimate tensile strength (Rm), fatigue resistance, and wear resistance. Decreases machinability.
• P <0.04%: Increases Re, Rm, wear resistance, and machinability. Decreases resilience.
• S <0.04%: Harmful in steels. Increases machinability and hardness, decreases elongation, resilience, and Re.
• Si <0.50%: Deoxidation, increasing Re, Rm, hardness, and wear resistance. Decreases machinability, resilience, and forgeability.

Influence of Alloy Elements

1. Mechanical Properties: Strengthening through training sequence and carbide precipitation.
2. Hardenability improvement.
3. Eutectoid temperature shifted downward, and less with higher %C alloy.
4. Grain Size: Tungsten and vanadium hamper growth.
5. Increased corrosion.
6. Raises tempering temperature.
7. Ms and Mf Temperatures: The greater the % alloy, the lower Ms and Mf. If very low, austenite can remain unprocessed.
8. Hotspots:
   • Gamma-stabilizers: Stabilize austenite, lower critical points (Ni, Mn, C, N, Co, Cr).
   • Alpha-stabilizers: Large ferrite field, lines A3 and ACM rise, raise hotspots (Ti, Mo, Si, W).
   • Chromium is alpha-stabilizing <12% for more C; gamma-stabilizing >12% for less C.
9. Dissolved in ferrite (solid solution): Ni, Al, Si, Cu, Co, P.
10. Forming carbides: Cr, Mn, Mo, V, W, Ti.

Hardening and Tempering Steel

Alloyed: Chromium always improves hardenability.

Unalloyed: Properties depend on %C. Good hardness, toughness, elastic limit, and Rm.

Stainless Steel

High alloy special. Pre-oxidized with a protective chromium oxide layer. Alloy elements: Cr (10.5%), Ni, Mo, Ti, Nb. Carbon (0.03 - 1.20%).

Ferritic Stainless Steel

Carbon <0.08%, contains no gamma-stabilizing elements, not hardened by quenching. Body-centered cubic (BCC) structure. Tenacious, tough, corrosion-resistant, low yield strength, economical. Very sensitive to grain growth. Can be purified by standard methods, hardened by cold deformation, and left with a high degree of strength (thermomechanical treatment).

Martensitic Stainless Steel

Formed by martensite at ambient temperatures but without tempering. Carbon <1.2%. Good hardness and strength. Low corrosion resistance. Used in cutting tools.

Austenitic Stainless Steel

Austenite at ambient temperature. Extends the austenite region through Ni and Cr, non-magnetic. No allotropic transformation, no refined grains. Very tough, good cold deformation. Disadvantage: Intergranular corrosion due to chromium carbide precipitation at the edge of the austenite grain. Solved using solution annealing or stabilized austenitic stainless steel (with Ti or Nb).

Austeno-Ferritic Stainless Steel

Austenite and ferrite. High %Cr, minimum 21%. Low %Ni. Better properties than austenitic, better tensile yield strength, and strength. High resistance to intergranular corrosion and low stress. Expensive.