What is Coefficient of Thermal Expansion (CTE) & calculation in welding?


What is Coefficient of Thermal expansion or CET?

The coefficient of thermal expansion is a measure (change in size over a defined size at a certain temperature) of how much a material expands during heating and contracts during cooling. The higher the coefficient of thermal expansion the more a material is going to expand and contract for a given temperature change. Therefore, the higher the coefficient of thermal expansion, the higher the distortion susceptibility.

THE Coefficient of linear thermal expansion (CTE, a, or a1) is a material property that is indicative of the extent to which a material expands upon heating. Different materials expand by different amounts at varying temperatures. Over little temperature ranges, the thermal extension of uniform straight objects is corresponding to temperature alter. Thermal extension finds valuable application in bimetallic strips for the development of thermometers but can create hindering inside stretch when a basic portion is heated and kept at a steady length.

Understanding Coefficient of Thermal Expansion

Most solid metals grow upon heating and contract when cooled. The alter in length with temperature for a solid metal can be communicated as:

where l0 and lf show, respectively, the original and final lengths with the temperature alter from TtoTf.  The parameter aCTE and has units of reciprocal temperature (K–1) such as µm/m · K or 10–6/K. 

How To Measure Coefficient of Thermal Expansion

To find the thermal coefficient expansion, two physical quantities (displacement and temperature) must be measured on an specimen that is undergoing a thermal cycle. Three of the main techniques used for CTE measurement are

1. Dilatometry,

2. Interferometry, and

3. thermomechanical analysis.

Optical imaging can too be utilized at extraordinary temperatures. X-ray diffraction can be utilized to ponder changes within the cross-section parameter but may not compare to bulk heat development.

Coefficient of Thermal Expansion in Steels

Simple chromium stainless steel material has a coefficient comparable to carbon steel, but that of the austenitic stainless steel material is almost 11⁄2 times higher.

This combined high thermal expansion and lower thermal conductivity for these stainless steel require utmost care to avoid adverse effects during welding.

E.g., while welding of austenitic stainless steel it is recommended to use low heat input, copper bars to dissipate welding heat, and proper welding sequence. The CET must be considered in components that utilize a blend of materials such as heat exchangers with mellow steel shells and austenitic steel tubes. Check the table at the conclusion to discover CTE for all materials.

Coefficient of Thermal Expansion in Welding

The CTE is an important parameter when welding dissimilar materials. Large differences in the coefficient of thermal expansion values of nearby materials during cooling shall create tensile stress in the first material and compressive stress in the second material. The material facing this tensile stress can get a hot crack during welding, or can cold crack during service if these residual stresses are not relieved. an example of such cases in refineries where weld of carbon steel and copper alloy are used together.

The most common problem in dissimilar metals welding (DMW) is because of the differences in the physical properties (mainly the coefficient of thermal expansion and heat conductivity) of the base materials as well as on certain levels of metallurgical incompatibility. The weld thermal cycle produces a differently featured heat-affected zone (HAZ), accompanied by microstructural changes in the engineered microstructure, which may lead to an important loss of joint quality.

Coefficient of thermal expansion for metal

The coefficient of thermal expansion of mostly used materials in the welding is given in the below table. For the coefficient of all materials, Click here to download this full table.

MaterialCTE (10-6/K )CTE (10-6/°F)
PureTungsten (W)4.5–4.6 2.5–2.6
Iron-cobalt-nickel alloys 0.6–8.70.3–4.8 
Pure Chromium (Cr)4.9–8.2 2.7–4.6
Pure Titanium (Ti) 8.4–8.6 4.7–4.8
Molybdenum alloys 4.0–14 2.2–7.8
Ferritic stainless steel 9.3–12 5.2–6.5 
Cast ferritic stainless steel 115.9
Martensitic stainless steel 9.5–12 5.3–6.6 
Iron carbon alloys 10–12 5.6–6.5 
Wrought iron 116.4
Structural steel 126.5
Nickel chromium molybdenum alloy steel 10–13 5.7–7.3
High-manganese carbon steel 11–13 6.2–7.0
Malleable cast iron 10–14 5.6–7.6 
Ductile medium-silicon cast iron 11–14 6.0–7.5
Gray cast iron 11–15 6.0–8.5
Austenitic stainless steel 9.8–25 5.4–14

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