Variables Which Affect Notch Toughness
The ferritic steels (carbon, low alloy, and 400 series stainless) undergo a ductile-to-brittle transition as temperature is lowered. Each of these steels has a ductile-to-brittle transition temperature range. Above their transition temperature range these steels are tough; in and below the transition temperature range, they can fracture in a brittle manner. Section 523 discusses transition temperature in more detail.
Factors that affect transition temperature of steels are:
1. Composition—Carbon content has the most effect, and transition temperature decreases (toughness improves) with decreased carbon content. Increasing manganese content contributes to a lower transition temperature up to a manganese content of about 2 wt %.
2. Deoxidation practice—Fully killed (fully deoxidized) steels have lower transition temperatures than semikilled or rimmed steels.
3. Grain size—Fine grained steel gives a lower transition temperature.
4. Heat treatment—Normalized or quenched and tempered steels have lower transition temperature ranges than as-rolled steels of similar composition. Grain refinement is a reason.
5. Welding—Welding generally results in a higher transition temperature in the weld heat-affected zones as compared to the base material. The variables which improve transition temperature in the base metal, as listed above, also help in the heat-affected zone. High carbon content is particularly detrimental because it causes harder, more brittle heat-affected zones.
Most brittle fractures initiate near welds. Stress concentrations at the weld toe, weld defects, and residual stresses are more often causes than is poor heataffected zone notch toughness. Postweld heat-treated structures generally have better brittle fracture resistance.
6. Embrittlement phenomena—Certain metallurgical phenomena are damaging to specific alloys. Temper embrittlement of 2¼ Cr-1 Mo steel after exposure at 650°F to 1000°F is one example. Special guidelines have been developed for fracture prevention of thick 2¼ Cr-1 Mo hydroprocessing reactors which operate in the embrittlement range. Some 400 series stainless steels suffer “885°F embrittlement” in the 650°F to 900°F range. Both temper embrittlement and “885°F embrittlement” cause an unfavorable upward shift of the transition temperature, such that the embrittled alloys have poor notch toughness at atmospheric temperature.
The toughness of low strength steels decreases with increasing loading rate. At a given temperature, the toughness measured in an impact test is lower than the toughness measured in a static test. Figure 500-12 shows a schematic representation of shift in transition temperature due to loading rate. The magnitude of the shift depends on the yield strength of the material.
The fracture toughness of a particular material decreases with increasing section thickness for two reasons. First, it is metallurgically more difficult to obtain good toughness properties as thickness increases. Second, thicker sections produce greater constraint ahead of the notch due to a triaxial state-of-stress. Beyond some limiting thickness, maximum constraint is obtained (called plane strain), and notch toughness approaches a minimum value (KIc). Thin materials have a biaxial state-of-stress (called plane stress), so have less constraint to plastic flow and act in a more ductile manner.
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