The sub-ambient temperature dependence of yield strength σo (Rp0.2) and ultimate tensile strength σu in a bcc metal is shown in Figure 2.2 below.
Figure 2.2 Low temperature behavior of metals.
The presence of a small flaw raises the NDT of steel by about 200°F (110°C). Increasing the flaw size decreases the fracture stress curve, as in curve EF, until with increasing flaw size a limiting curve of fracture stress HJKL is reached. Below the NDT the limiting safe stress is 5,000 to 8,000 psi (~35 to 55 MPa).
Above the NDT the stress required for the unstable propagation of a long flaw (JKL) rises sharply with increasing temperature. This is the crack-arrest temperature curve (CAT). The CAT curve defines the highest temperature at which unstable crack propagation can occur at any stress level. Fracture will not occur for any point to the right of the CAT curve.
The temperature above which elastic stresses cannot propagate a crack is the fracture transition elastic (FTE). The temperature defines the FTE, at the point K, when the CAT curve crosses the Yield Strength, σo curve. The fracture transition plastic (FTP) is the temperature where the CAT curve crosses the Ultimate Tensile Strength σu curve (point L). Above this temperature the material behaves as if it is flaw-free, for any crack, no matter how large, cannot propagate as an unstable fracture.
2.1.4.1 Metal Strength at Low Temperature
As we have seen as temperature is lowered from room temperature 75oF (24oC or 297oK) to absolute zero 1oK the atoms of an element move closer together by dimensions easily compounded from the coefficient of thermal expansion. Number of changes occurs as a result of this smaller lattice parameter. For example, the elastic module increases. In general the tensile strength and yield strength of all materials increase as the temperature is lowered to the extent that at NDT the yield and Tensile strength are equal (σo = σu). The change in these properties is variable in degree for different metals but change does occur.
When the temperature of low carbon or low alloy steel is lowered the corresponding increase in strength of metals is attributed to an increase in resistance to plastic flow. Since plastic flow is strongly dependent upon the nature of the crystalline structure, it would be logical to assume that metals with the same kind of structure would react in similar manner.
Because toughness tends to decrease as temperatures are lowered, especially for bcc-structured material like steel. Testing is often carried out to measure and monitor this property of steel. The most frequently used test specimen is notched-bar impact specimen, despite the shortcomings of this test. The popularity of impact test is due to its long established position in standards and relatively easy procedure available with laboratories to test standard Charpy V-notch specimen.
2.1.5 Elevated Temperature Properties
The behavior of metal at the elevated temperature assumes importance for primarily following reasons.
1 1. Welding operation involves heat, and higher temperature, it is essential that we understand the changes in the metal’s mechanical properties, and predict its behavior under induced strain during welding. Going through the cycle of heating and cooling can significantly alter the properties of base metal, and the weld itself.
2 2. Metals are formed and shaped to be useful for the specific structural applications. It is important that the metal so formed and worked on possess the desired. During hot working the metal may develop flaws that may subsequently reduce the strength of the structure.
3 3. May weldments are exposed to high temperature service.
Strength of the metal reduces as the temperature rises, the elastic modulus is reduced, and plastic deformation is more active.
A weldment during welding experiences rapid rise of temperature to its melting point, and then it rapidly cools. If the structure or its any member is restrained the hot weld metal and the HAZ area goes through complex thermal strains. This can lead to weld or HAZ or both cracks, or distortion occurs in the structure, due to the shrinkage stress.
Apart from the normal strength and ductility parameters are not very effective in predicting the behavior of metal at elevated temperature. Other properties such as time at the elevated temperature, and the applied stress give a new parameter to consider, the creep property of the metal. The metal at high temperature and under stress continues to creep over the time to its failure, the time is an important factor here.
2.1.6 Physical Properties
In the following paragraphs we discuss some of the physical properties of metals that have significant impact on the welding and weldments.
2.1.6.1 Thermal Conductivity
Thermal conductivity is one of the physical properties of the metal. For a welding engineer this is an important property to know and understand about the metal they intend to use for welding. Metals are a good heat conductors and they transmit (welding) heat away from the heat source, some metals conduct heat much faster than the other; for example, Aluminum and Copper conduct heat much faster than steel. This difference in thermal conductivity explains the varying degree of weldability and use of low heat welding process for Aluminum and Copper and their alloys.
2.1.6.2 Coefficient of Thermal Expansion
Metals when heated, expand in volume. The coefficient of thermal expansion is the liner measure of this expansion. It give a unit change in the liner dimension per unit increase in temperature. Note that the term used in the description above is change, because coefficient of thermal expansion also gives the change in dimension as contraction as the metal cools. But the expansion and contraction with the change in the temperature is not just liner, it is volumetric. Engineering data and table only give the Liner values as coefficient of thermal expansion.
The volumetric expansion is of interest to welding engineers, and fabricators, because the expansion and subsequent cooling causes shrinkages and distortions in the structure. A welding engineer is concerned about these changes in dimension and provides solutions to avoid, or eliminate the effect of thermals effect on the dimension of the weldment.
2.1.6.3 Melting Point
Welding is an activity significantly aligned with melting of metals by application of heat. Elemental metals have different melting points, and alloy metals have different range of melting temperature. Higher the melting temperature larger amount of heat is required to melt the metal being welded. This requires proper selection of welding process by the welding engineer.
If however joining is planned by Brazing or soldering then too, selection of heat source assumes importance because the heat source should be able to melt the brazing rod to wet both joining surfaces.
For welding two different metals that have significantly differing melting points or ranges becomes a challenge to a welding engineer, a well-considered heat source is necessary and welding procedure should be developed to regulate application and control of welding heat.
2.1.7 Electrical Conductivity
Electrical conductivity is an important property to consider if electric resistance welding is the process used for joining two metals. Metals are relatively good electrical conductors, however