DISPOSICIONES PRELIMINARES Capítulo

All solution treatments gave a lath martensite structure in both

alloys, as seen in Figs. 109-114. The grain sizes vary with

austenitizing treatment and molybdenum content and are tabulated in Table 12 together with the quantitative analyses of the inclusion contents of the alloy. Figs. 116-119. A deviation from the fully martensite condition was observed in the higher molybdenum alloy after the highest solution treatment temperature of 1300°C. Here, delta ferrite was seen to form at the prior austenite grain boundaries. This was due to molybdenum being a ferrite former and the high temperature favouring the formation of delta ferrite, Fig. 114.

The analysis of the inclusions found in the steel was carried out on the microprobe at Harwell. The results are shown in Table 13 and Figure 115 demonstrating how the inclusions are two phase, a manganese oxide at the centre and a manganese sulphide surrounding it.

5.2.2 General Discussion of the Effect of Molybdenum on the As Quenched Properties of the Fe-8Mn Alloy

The change of the brittle fracture mode from intergranular to transgranular cleavage with the addition of 2.5% molybdenum to the Fe-8Mn coincided with a lowering of the ductile to brittle transition temperature. This is in agreement with work by Squires, et al, on the

123

effect of molybdenum additions on Fe-Ni-Mn alloys. One explanation

of this is to consider the effect of molybdenum on other elements in solid solution. Nazim concluded that manganese, phosphorus and nitrogen segregated to grain boundaries during austenitisation and nitrogen segregation occurred on cooling,when martensite was formed,to cause intergranular failure. Therefore, it may follow that molybdenum in the Fe-Mn-Mo alloys acts in some way to suppress these effects. The molybdenum immobilises the manganese, phosphorus and nitrogen. The mechanism of this immobilisation has not yet been determined but it has

long been assumed to be a precipitation reaction such as the formation of Mo^P or MoN. However, recently Wada has reported that molybdenum

124

suppresses the grain boundary segregation of phosphorus but the

attractive interaction is not strong enough to form stable Mo-P pairs in ferrite. The tendency of these elements to stay in solid solution could then be due to the increase of the solubility of phosphorus, manganese and nitrogen in iron, although it has been reported that

125 molybdenum lowers the solubility of phosphorus in iron.

However, the former is more likely since molybdenum is a IT - loop element which increases solid solubility in ferrite.

115

Homdros and Seah described the relationship between solid

solubility and the interaction of solute and solvent. In empirical terms the lower the solid solubility, the smaller the amount ofelements in the matrix. This gives rise to a strong solute/solvent interaction and

the elements are more likely to segregate. Conversely, the higher the solid solubility the larger the amount in the matrix. The solute/ solvent interaction will be weaker and so the elements will be less likely to segregate.

It has also been reported that molybdenum can increase the

activation energy for phosphorus diffusion in a Ni-Cr steel. So there may be possible similar interaction between molybdenum and manganese and between molybdenum and nitrogen. The increase in activation energy would result in a slower diffusion of these elements to grain boundaries.

Unlike the work done by Squires, the addition of a further 2.5% molybdenum to the alloy, making a total of 5%, does not decrease the

ductile to brittle transition temperature still further. This is unusual, since the alloy containing 5% molybdenum is softer than the 2.5% alloy and the corresponding ductile to brittle transition temperature should then be lower. It is also seen that the 2.5% molybdenum alloy is harder than the base alloy but the ductile to brittle transition

temperature is lower due to a change from intergranular to brittle cleavage fracture i.e. a suppression of embrittlement.

The existence of an optimum amount of molybdenum that can suppress 86 128

embrittlement has been known about for a long time r . It may be

that the optimum amount was reached in these alloys between 2.5% and 5% and so it will not increase the toughness further.

The hardness of the as-quenched alloys increases in the order K1525 (0% Molybdenum), K1527 (5% Molybdenum) K1526 (2.5% Molybdenum). Hardness

depends on tensile strength rather than yield strength. Earlier work

by Buick showed uniform increase in yield strength with molybdenum content from 0 - 3 - 5% in an Fe-Mo alloy whereas tensile strength

126 increases in the order Fe - Fe5%Mo - Fe3%Mo.

This effect could be due to a phenomenon known as alloy or solid 127

solution softening. This can be explained in two ways:-

1. The extrinsic theory in which the starting b.c.c. metal would be hardened by residual impurities and a scavenging of these impurities by substitutional atoms would induce softening e.g. clustering of nitrogen around molybdenum.

2. The intrinsic theory in which the low temperature yield stress of b.c.c. metals and alloys would be controlled by a lattice

friction or Peierls stress on the screw dislocations and the

softening would correspond to a decrease of the lattice friction strength in the dilute alloys.

As well as the effect on the ductile to brittle transition temperatures, additions of molybdenum are also seen to affect the upper and lower shelf energies. An addition of 2*5% molybdenum to the Fe-8Mn alloy lowers the upper shelf energy for ductile fracture from

72J to ~ 42J. A further 2.5% molybdenum addition (5% total) increases the upper shelf to ~ 60J. Two further observations may be related to this effect. The first is that the inclusion content is higher in the alloy containing the lower level of molybdenum. The second is that the hardness increases in the order

K1525 (0% Mo) K1527 (5% Mo) - K1526 (2.5% Mo)

258 Hv 30 284 Hv 30 336 Hv 30

An increase in hardness means anincrease in yield stress and,

therefore, a lower energy of fracture. Therefore, the 5% molybdenum alloy can be expected to have a lower energy for ductile fracture than the base alloy, as is observed, and the 2.5% molybdenum alloy a lower energy for ductile fracture than that observed in the alloy containing 5% molybdenum, again as observed.

The ductile dimple size is smaller ( *>* lOjum) in the 2.5% molybdenum alloy compared to 3^wn in the alloy containing 5% molybdenum. A finer dimple size will give a higher upper shelf

energy because a greater amount of tearing is involved and so more energy needed per unit area to propagate fracture.

The lower shelf energy increases in the order K1525 (base) - K1527 (5% Mo) - K1526 (2.5% Mo). This is more difficult to explain. The base alloy fails by intergranular fracture in the brittle fracture mode. The two other alloys, with additions of molybdenum fail by transgranular cleavage in the brittle fracture mode. The energy required for the latter can be expected to be higher. This does not explain the fact that an alloy with a higher addition of molybdenum has a lower value of brittle fracture energy than the alloy with the lower addition of molybdenum. It is noted here that the hardness of K1526 (2.5% molybdenum) has a higher hardness than the alloy containing 5% molybdenum. The same cause of this higher hardness may also inhibit cleavage crack propagation and have the effect of a higher energy