Summary on steels

Iron exists as alpha, gamma, delta; these have widely different carbon solubilities

Iron-carbon phase diagram shows how steels differ, how they are heat treated

P (two phases) comes from AUS - diffusion process. Time dependent as shown by TTT curves

Hypoeutectoid steels are proeut. FE & P. Amt. of P determines mech. props.

Hypereutectoid steels are proeut. CM & P

Quenching AUS gives MAR by shear process, not diffusion. MAR is metastable. C is trapped in wrong iron structure, giving extremely hard, extremely brittle mat'l.

MAR transformation is temperature dependent (ms to mf)

Harden = make MAR. Hardenability = ability to make MAR

Hardenability depends on quench rate, position of TTT curves. Most alloy elements, increased C, increased AUS grain size move TTT curves to right, giving increased hardenability

Rapid quenching may result in distortion, quench cracks

MAR is almost always tempered. Tempering gives stress relief, starts carbide and FE formation

Jominy curves measure hardenability

Quench severity depends upon medium, agitation

Curves for bar diam vs Jominy distance, at various quench severities, for different hardening depths, exist

Can use these curves to design quenching processes

Hypoeutectoid steels - austenitize above A3

Hypereutectoid steels - austenitize above A1

Tempering at low T gives high hardness, low toughness - wear resistance

Tempering at hi T gives high toughness, reduced hardness

Increased tempering time gives decreased YS, TS, increased EL

Stainless steels have hi Cr content

Chromium oxide coating gives oxidation, corrosion resistance

Cr has infinite solubility in alpha-Fe, limited solubility in gamma-Fe (see PDs)

Ni has infinite solubility in gamma-Fe, limited solubility in alpha-Fe

Thus, 3 classes of SS: ferritic (Cr>14%), martensitic (12-13%Cr); austenitic (Cr>18% & Ni>8%)

Only martensitic SS hardenable

Austenitic steels have high strain hardening exponents (stress = constant x strain^n)

Tool steels: high C and/or high alloying element content ...... some high enuf to be air hardenable

(Carbide tools not tool steels. Usually WC in Co, fabricated by p/m methods. Very high hardness, wear resistance)

(Ceramic tools: usually Al2O3, pressed and sintered. Hardness, wear resistance better than carbide tools, but brittle, costly)

Cast irons are about 2.5-5%C

White cast irons have all carbon as CM. Brittle, limited use

Grey cast irons have part of C as graphite flakes. Flakes are stress raisers, so mat'l is brittle.

Grey cast irons extensively used. Cheap. Vibration damping by graphite flakes

Ductile cast irons have part of C as nearly spherical nodules. This gives ductility

Properties of cast irons controlled by graphite particle shape

In case hardening, region near surface is made harder than core. Usually for wear resistance

Can change hardenability at surface by increasing carbon content. Done by pack or gas carburizing (CO). Ht treat after carburizing

Can also form hard case by nitriding (NH3). Forms hard iron nitride, no ht treat req'd

Carbonitriding (CO and NH3) increases surface C concentration & forms nitride

Induction hardening: case formed by induction heating of surface only, then rapid quenching to form MAR