ASM Metals HandBook Volume 12 - Fractography
857 pág.

ASM Metals HandBook Volume 12 - Fractography

DisciplinaMetalurgia Mecânica151 materiais1.917 seguidores
Pré-visualização50 páginas
Effect of Temperature. Depending on the material, the test temperature can have a significant effect on the fracture 
appearance and in many cases can result in a change in the fracture mode. However, for materials that exhibit a phase 
change or are subject to a precipitation reaction at a specific temperature, it is often difficult to separate the effect on the 
fracture due to the change in temperature from that due to the solid-state reactions. In general, slip, and thus plastic 
deformation, is more difficult at low temperatures, and materials show reduced ductility and an increased tendency for 
more brittle behavior than at high temperatures. 
A convenient means of displaying the fracture behavior of a specific material is a fracture map. When sufficient fracture 
mode data are available for an alloy, areas of known fracture mode can be outlined on a phase diagram or can be plotted 
as a function of such variables as the test temperature and strain rate (Fig. 68). Similar maps can also be constructed for 
low-temperature fracture behavior. 
Fig. 68 Possible fracture zones mapped for a 0.2% C plain carbon steel in strain rate temperature space. T, 
testing temperature; Tm, melting temperature. The zones, which are shaded in the diagram, are as follows: A, 
subsolidus intergranular fracture due to segregation of sulfur and phosphorus; B, high strain rate intergranular 
fracture associated with MnS; C, ductile intergranular fracture--may or may not be preceded by B or D; D, low 
strain rate intergranular fracture; E, two-phase mixture with fracture at second-phase particles in the weaker 
preferentially strained ferrite. Source: Ref 209 
Effect of Low Temperature. Similar to the effect of the state of stress, low temperatures affect the bcc metals far 
more than the fcc or hcp metal systems (see the section "Effect of the State of Stress" in this article). Although lower 
temperatures can result in a decrease in the size and depth of dimples in fcc and hcp metals, bcc metals often exhibit a 
change in the fracture mode, which generally occurs as a change from dimple rupture or intergranular fracture to 
cleavage. For example, a fully pearlitic AISI 1080 carbon steel tested at 125 °C (255 °F) showed a fracture that consisted 
entirely of dimple rupture; at room temperature, only 30% of the fracture was dimple rupture, with 70% exhibiting 
cleavage. At -125 °C (-195 °F), the amount of cleavage fracture increased to 99% (Ref 210). This transition in fracture 
mode is illustrated in Fig. 69. 
Fig. 69 Effect of test temperature on a fully pearlitic AISI 1080 carbon steel. Smooth cylindrical specimens 
tensile tested at a strain rate of 3.3 × 10-4 s-1. Specimens tested at 125 °C (225 °F) show fractures consisting 
entirely of dimple rupture (a), while at -125 °C (-195 °F), the fractures exhibit 99% cleavage (b). The size of 
the cleavage approximates the prior-austenite grain size. Source: Ref 210 
Charpy impact testing of an AISI 1042 carbon steel whose microstructure consisted of slightly tempered martensite (660 
HV) as well as one containing a tempered martensite (335 HV) microstructure at 100 °C (212 °F) and at -196 °C (-320 
°F) produced results essentially identical to those observed for the AISI 1080 steel. In both conditions, the fracture mode 
changed from dimple rupture at 100 °C (212 °F) to cleavage at -196 °C (-320 °F), as shown in Fig. 70. Similar changes in 
the fracture mode, including a change to quasi-cleavage, can be observed for other quench-and-temper and precipitation-
hardenable steels. 
Fig. 70 Effect of test temperature on an AISI 1042 carbon steel with a slightly tempered martensitic (660 HV) 
microstructure that was Charpy impact tested at -196 and 100 °C (-320 and 212 °F). The fracture at -196 °C (-
320 °F) consists entirely of cleavage (a), and at 100 °C (212 °F), it is dimple rupture (b). 
A unique effect of temperature was observed in a 0.39C-2.05Si-0.005P-0.005S low-carbon steel that was tempered 1 h at 
550 °C (1020 °F) to a hardness of 30 HRC and Charpy impact tested at room temperature and at -85 °C (-120 °F) (Fig. 
71). In this case, the fracture exhibited intergranular decohesion at room temperature and changed to a combination of 
intergranular decohesion and cleavage at -85 °C (-120 °F). This behavior was attributed to the intrinsic reduction in 
matrix toughness by the silicon in the alloy, because when nickel is substituted for the silicon the matrix toughness is 
increased and no cleavage is observed (Ref 211). 
Fig. 71 Effect of test temperature on a 0.39C-2.05Si-0.005P-0.005S steel that was heat treated to a hardness 
of 30 HRC and Charpy impact tested at room temperature at -85 °C (-120 °F). The fracture at room 
temperature occurs by intergranular fracture (a) and by a combination of intergranular fracture and cleavage 
(b) at -85 °C (-120 °F). Source: Ref 211 
The temperature at which a sudden decrease in the Charpy impact energy occurs is known as the ductile-to-brittle 
transition temperature for that specific alloy and strength level. Charpy impact is a severe test because the stress 
concentration effect of the notch, the triaxial state of stress adjacent to the notch, and the high strain rate due to the impact 
loading combine to add to the reduction in ductility resulting from the decrease in the testing temperature. Although 
temperature has a strong effect on the fracture process, a Charpy impact test actually measures the response of a material 
to the combined effect of temperature and strain rate. 
The effects of high temperature on fracture are more complex because solid-state reactions, such as phase changes 
and precipitation, are more likely to occur, and these changes affect bcc as well as fcc and hcp alloys. As shown in Fig. 
72, the size of the dimples generally increases with temperature (Ref 209, 212, 213). The dimples on transgranular 
fractures and those on intergranular facets in a 0.3C-1Cr-1.25Mo-0.25V-0.7Mn-0.04P steel that was heat treated to an 
ultimate strength of 880 MPa (128 ksi) show an increase in size when tested at temperatures ranging from room 
temperature to 600 °C (1110 °F) (Ref 213). 
Fig. 72 Effect of temperature on dimple size in a 0.3C-1Cr-1.25Mo-0.25V-0.7Mn-0.04P steel that was heat 
treated to an ultimate strength level of 880 MPa (128 ksi). (a) and (b) Dimples on transgranular facets. (c) and 
(d) Dimples on intergranular facets. Note that dimple size increased with temperature. Source: Ref 213 
Figure 73 shows the effect of temperature on the fracture mode of an ultralow-carbon steel. The steel, which normally 
fractures by dimple rupture at room temperature, fractured by intergranular decohesion when tensile tested at a strain rate 
of 2.3 × 10-2 s-1 at 950 °C (1740 °F). The change in fracture mode was due to the precipitation of critical submicron-size 
MnS precipitates at the grain boundaries. This embrittlement can be eliminated by aging at 1200 °C (2190 °F), which 
coarsens the MnS precipitates (Ref 209). 
Fig. 73 Effect of temperature on the fracture of an ultralow-carbon steel. The 0.05C-0.82Mn-0.28Si steel 
containing 180 ppm S was annealed for 5 min at 1425 °C (2620 °F), cooled to 950 °C (1740 °F), and held for 3 
min before tensile testing at a strain rate of 2.3 × 10-4 s-1. The fracture, which occurs by dimple rupture when 
tested at room temperature, exhibits intergranular decohesion at 950 °C (1740 °F). Source: Ref 209 
A similar effect was observed for Inconel X-750 nickel-base alloy that was heat treated by a standard double-aging 
process and tested at a nominal strain rate of 3 × 10-5 s-1 at room temperature and at 816 °C (1500 °F). The fracture path 
was intergranular at room temperature and at 816 °C (1500 °F), except that the room-temperature fracture exhibited 
dimples on the intergranular facets and those resulting from fracture at 816 °C (1500