Guangming Group Today’s Sharing: What Factors Affect the Thermal Expansion Coefficient of Powder Metallurgy Gears?

Main Factors Affecting the CTE of Powder Metallurgy Gears

The biggest difference between powder metallurgy (PM) parts and dense forged steel lies in porosity, followed by alloy phase composition, microstructure after heat treatment and manufacturing processes. Below is a prioritized summary, mainly for ferrous PM gears which are most widely used:

1. Chemical Composition and Alloy Phases (Core Factor)

Copper (Cu) has a much higher thermal expansion than iron. Increasing Cu content raises the overall CTE, and copper is a common element used to adjust expansion properties for oil pump gears.

Nickel (Ni), Molybdenum (Mo) and Chromium (Cr): Ni reduces thermal expansion (Invar alloy system). Mo and Cr are lowexpansion alloying elements. Small additions slightly lower CTE while improving hardenability.

Carbon (graphite): Carbon changes the phase constitution of the iron matrix. High carbon tends to form pearlite, while low carbon favors ferrite. Ferrite, pearlite and austenite have distinctly different CTE values.

Rule of mixtures for multiphase materials: The final CTE is roughly determined by the volume fraction weighting of each phase, e.g., mixed microstructure of ferrite + pearlite + free copper + graphite.

2. Porosity, Density and Pore Morphology (Unique Key Factor for PM)

Pores are voids filled with air and barely expand. For materials with identical composition, higher porosity and lower density result in a lower measured mean linear CTE.

Open pores and closed pores exert different degrees of influence. Uneven pore distribution leads to thermal expansion anisotropy.

Powderforged PM gears (nearfull density) have CTE values close to forged steel of the same composition, while conventional sintered porous parts show obviously lower CTE.

Example: For FeCuC ferrous materials, the CTE difference can reach 0.5~1.2×10⁻⁶/℃ between a density of 7.0 g/cm³ and 7.6 g/cm³.

3. Metallographic Microstructure and Phase Transformation

Ferrite, pearlite, bainite, martensite and austenite each have their own CTE. Martensitic transformation is accompanied by volume distortion.

When the temperature crosses transformation points (Ac1/Ac3), the thermal expansion curve changes abruptly, and a negative expansion range may appear. A single fixed CTE cannot be used for deformation calculation within this range.

Retained austenite is an easily overlooked factor for precision gears: retained austenite may transform with temperature or aging, causing expansion drift.

4. Manufacturing Processes

Sintering process: Sintering temperature, holding time and atmosphere govern the degree of alloying, pore size, grain size and copper infiltration effect. Copperinfiltrated gears feature a continuous copper phase, delivering higher CTE closer to dense alloys.

Compaction, repressing and sizing: These induce pore orientation and texture, resulting in thermal expansion anisotropy (radial and toothdirection expansion differ, affecting backlash).

Heat treatment (quenching, tempering, stabilization treatment): Modifies phase composition, residual stress and retained austenite. Residual stress overlaps with thermal deformation and makes measured expansion deviate from the intrinsic material CTE.

Oil impregnation / resin impregnation: Filling pores with foreign substances slightly changes CTE, which needs to be considered for precision transmission gears.

5. Test Conditions and Temperature Range

CTE is temperaturedependent rather than a constant. Values differ between room temperature range (20~100℃) and high temperature range (100~200℃). CTE data shall be selected according to the actual operating temperature range of gears during selection.

Heating rate and sampling location (density difference between tooth portion and hub) cause test scatter. Fluctuation inherently exists among PM gears from the same batch.

Practical Engineering Summary for Gear Mating Design

Typical CTE of ferrous PM gears: 11~12.5×10⁻⁶/℃. Higher Cu content can lift it to around 12.8×10⁻⁶/℃ to match mating steel gears. Highporosity, lowdensity grades have lower CTE; nearfulldensity powderforged grades approach forged steel at ~13×10⁻⁶/℃.

Do not directly adopt forged steel parameters for clearance design. Prioritize measured CTE tested under the target temperature range, with the same density and composition.

Keywords: powder metallurgy, metal injection molding, powder metallurgy gears

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