Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range

Importancia y uso:

5.1 The property T₀ determined by this test method characterizes the fracture toughness resistance of ferritic steels to cleavage cracking. T₀ is determined by statistical analysis of sets of fracture toughness data. Historically, estimation of fracture toughness resistance to cleavage cracking has been based on correlative testing of Charpy and nil-ductility specimens, or by conducting extensive testing over a wide temperature range and then subjectively characterizing the ductile to brittle transition curve. The statistical methods detailed in this test method eliminate the need for a correlative approach and reduce this subjectivity and the number of specimens necessary to characterize the fracture toughness resistance of ferritic steels to cleavage cracking.

5.2 Shifts in T₀ are a measure of transition temperature change caused, for example, by metallurgical damage mechanisms.

5.3 Ferritic steels are microscopically inhomogeneous with respect to the orientation of individual grains. Also, grain boundaries have properties distinct from those of the grains. Both contain carbides or nonmetallic inclusions that can act as nucleation sites for cleavage microcracks. The random location of such nucleation sites with respect to the position of the crack front manifests itself as variability of the associated fracture toughness (9). This results in a distribution of fracture toughness values that is amenable to characterization using the statistical methods in this test method.

5.4 The statistical methods in this test method assume that the data set represents a macroscopically homogeneous material, such that the test material has both uniform tensile and fracture toughness properties. The fracture toughness evaluation of nonuniform materials is not amenable to the statistical analysis procedures employed in this test method. For example, multi-pass weldments can contain heat-affected and brittle zones with localized properties that are quite different from either the bulk or weld materials. Thick-section steels also often exhibit some variation in properties near the surfaces. Metallographic analysis can be used to identify possible nonuniform regions in a material. These regions can then be evaluated through mechanical testing such as hardness, microhardness, and tensile testing for comparison with the bulk material. It is advisable to measure the toughness properties of these nonuniform regions distinctly from the bulk material. Section 10.6 provides a screening criterion to assess whether the data set may not be representative of a macroscopically homogeneous material, and therefore, may not be amenable to the statistical analysis procedures employed in this test method. If the data set fails the screening criterion in 10.6, the homogeneity of the material and its fracture toughness can be more accurately assessed using the analysis methods described in Appendix X5.

5.5 Distributions of K_Jc data are characterized in this test method using a Weibull function that is coupled with weakest-link statistics (10). An upper limit on constraint loss and a lower limit on test temperature are defined between which weakest-link statistics can be used. For some materials, particularly those with low strain hardening, the value of T₀ may be influenced by specimen size due to a partial loss of crack-tip constraint (1). When this occurs, the value of T₀ may be lower than the value that would be obtained from a data set of K_Jc values derived using larger specimens.

5.6 There is an expected bias among T₀ values as between SE(B) and C(T)/DC(T) specimen types. The magnitude of the bias may increase inversely to the strain hardening ability of the test material at a given yield strength, as the average crack-tip constraint of the data set decreases (11). On average, T₀ values obtained from C(T) specimens are higher than T₀ values obtained from SE(B) specimens. Best estimate comparison indicates that the average difference between C(T) and SE(B)-derived T₀ values is approximately 10 °C (12). However, individual C(T) and SE(B) datasets may show much larger T₀ differences (13, 14, 15), or the SE(B) T₀ values may be higher than the C(T) values (12) . On the other hand, comparisons of individual, small datasets may not necessarily reveal this average trend. Datasets which contain both C(T) and SE(B) specimens may generate T₀ results which fall between the T₀ values calculated using solely C(T) or SE(B) specimens.

Subcomité:

E08.07

Referida por:

E0636-20, E0185-21, E1253-21, E2215-24, E1823-24C, F3439-22, E0399-24, E1820-25A, E0509_E0509M-21, E2899-24E01

Volúmen:

03.01

Número ICS:

77.040.10 (Mechanical testing of metals)

Palabras clave:

ductile-to-brittle transition; fracture toughness; ferritic steels; master curve; T0;