Factors affecting brittleness of steels

7.4.4 Factors affecting brittleness of steels

Many of the effects of alloying, heat treatment and condition of testing on brittle fracture can be rationalized on the basis of the above ‘transition’ equation.

7.4.4.1 Ductile–brittle transition

Under conditions where the value of the left-hand side of equation (7.28) is less than the value of the right-hand side, ductile behavior should be observed; when the left-hand side exceeds the right-hand side the behavior should be brittle. Since the right-hand side of equation (7.28) varies only slowly with temperature, it is the way in which changes occur in values of the terms on the left of the equation which are important in determining the ductile–brittle transition. Thus, in a given material brittleness should be favored by low temperatures and high strain rate, because these give rise to large values of σ i and k y , and by large grain sizes. On the right-hand side, the typical effect of a sharp notch is to raise the transition temperature of structural steel from around 100 K for a normal tensile test into the range of 200–300 K, because the value of β is lowered.

7.4.4.2 Effects of composition and grain size

At a constant temperature, because the values of σ i and k y remain fixed, the transition point will occur at a critical grain size, above which the metal is brittle and below which it is ductile.

The inclusion in equation (7.28) of the grain-size term, d 1/2 , in combination with the σ i , term, enables many previous metallurgical misunderstandings to be cleared up. It shows that there is no simple connection between hardness and brittleness, since hardening produced by refining the grain size reduces the brittleness, whereas hardening due to an increase in σ i increases the brittleness.

Heat treatment is generally used to control the grain size of the sample and refine the structure. ‘Killed’ steel has very good notch toughness, because aluminum additions refine the grain size. Man- ganese reduces the grain size and by combining with carbon also reduces the k y value, so that this addi- tion is especially beneficial in improving low-temperature ductility. It is fairly evident that an improved notch toughness steel, compared with that used for welded ships in World War II, is given by increas- ing the manganese content and decreasing the carbon content, i.e. a high manganese-to-carbon ratio. Other additions, particularly nickel and chromium, have a similar effect on low-temperature ductility.

The Group 6A metals (Cr, Mo and W) are more susceptible to brittle fracture than the Group 5A metals (V, Nb and Ta). A comparison of these metals in terms of cleavage fracture is difficult, however, since Cr, Mo and W are susceptible to grain boundary fracture because segregation of impurities to such regions reduces the effective surface energy γ. However, even if this effect is eliminated by lowering the impurity level, it appears that Ta, Nb and V are more ductile than Fe, Mo, Cr and W, presumably because they have a lower k y /μ ratio and a higher γ value.

432 Physical Metallurgy and Advanced Materials

7.4.4.3 Work hardening and irradiation hardening

Small amounts of plastic deformation at room temperature, which overcome the yield point and unlock some of the dislocations, improve the ductility at low temperatures. The room-temperature ductility of chromium is similarly affected by small amounts of plastic deformation at 400 ◦

C. In general, however, plastic deformation which leads to work hardening embrittles the metal because it raises the σ i contribution, due to the formation of intersecting dislocations, vacancy aggregates and other lattice defects.

The importance of twins in fracture is not clear as there are several mechanisms other than twinning for the formation of a crack which can initiate fracture, and there is good evidence that microcracks form in steel in the absence of twins and that cracks start at inclusions. Nevertheless, twinning and cleavage are generally found under similar conditions of temperature and strain rate in bcc transition metals, probably because both phenomena occur at high stress levels. The nucleation of a twin requires

a higher stress than the propagation of the twin interface. Irradiation hardening also embrittles the metal. According to the theory of this type of hardening

outlined in Chapter 6, radiation damage can produce an increase in both k y (migration to dislocations of vacancies or interstitials) and σ i (formation of dislocation loops and other aggregated defects). However, for steel, radiation hardening is principally due to an increased σ i contribution, presumably because the dislocations in mild steel are already too heavily locked with carbon atoms for any change in the structure of the dislocation to make any appreciable difference to k y . Nevertheless, a neutron dose of 1.9

× 10 23 nm −2 will render a typical fine-grained, unnotched mild steel, which is normally ductile at −196 ◦

C, quite brittle. Moreover, experiments on notched fine-grained steel samples (see Figure 7.1c) show that this dose increases the ductile–brittle transition temperature by 65 ◦ C.

7.4.4.4 Microstructure

The change in orientation at individual grain boundaries impedes the propagation of the cleavage crack by (1) creating cleavage steps, (2) causing localized deformation and (3) tearing near the grain boundary. It is the extra work done (γ p ) in such processes which increases the apparent surface energy γ to (γ s +γ p ). It follows, therefore, that the smaller the distance a crack is able to propagate without being deviated by a change of orientation of the cleavage plane, the greater is the resistance to brittle fracture. In this respect, the coarser high-temperature products of steel, such as pearlite and upper bainite, have inferior fracture characteristics compared with the finer lower bainite and martensite products. The fact that coarse carbides promote cleavage while fine carbides lead to ductile behavior has already been discussed.