Heat Treating of Powder Metallurgy Steels,Heat Treating of Powder Metallurgy Steels,Heat Treating of Powder Metallurgy Steels
Introduction
ATTAINING CONSISTENT RESULTS has always been a challenge in the heat treatment of sintered powder metallurgy
(P/M) steel components. Unlike hardenability in wrought steel, where the response is controlled primarily by chemistry
and grain size, hardenability in P/M steel is significantly influenced by interconnected porosity. Interconnected porosity is
a network of connecting pores in a sintered object that permits a fluid or gas to pass through the object; also referred to as
interlocking or open porosity. Because P/M is a net-shaped process, dimensional change or distortion occur that may
affect the function or fit of the part. If a P/M part has been slack quenched, to reharden it without degrading the strength
and toughness of the part is difficult.
In P/M, hardenability is influenced not only by porosity but also by prior processing variables such as graphite
segregation and the degree of sintering. When P/M parts are quenched from the austenitic range, the external surfaces
may be exposed to much higher cooling rates than are the interior surfaces, resulting in mixed microstructures and
inconsistent hardness readings. These variations increase with the part design complexity. The next section briefly
reviews the relationship between porosity and properties.
Effect of Porosity on Material Properties
The mechanical properties of wrought, fully dense steel are determined by chemical composition, microstructure, and
surface characteristics. When porosity is uniformly distributed throughout a metal, the response to external stress can
affect other physical, mechanical, and electrical properties. The degree of porosity in a metal affects the following
material properties:
· Density
· Conductivity (thermal and electrical)
· Magnetic permeability
· Saturation induction
· Tensile strength, yield strength, and elongation
· Fatigue impact strength
· Modulus of elasticity
· Poisson's ratio
· Apparent hardness

· Hardenability
· Surface roughness
Approximate mathematical relationships [per Metal Powder Industries Federal (MPIF) Standard 35] between porosity and
some physical properties are expressed as:
(Theoretical density of metal) x 1-%porosity
100
Density = æç ö÷
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(Theoretical conductivity of metal) x 1-2%porosity
100
Conductivity = æç ö÷
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Fatigue strength
= 0.40 × U.T.S., determined experimentally
Modulus of elasticity of sintered steel (Modulos of elasticity of wrought steel) x 1-%porosity , valid when % porosity < 50

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