General Discussion

The objective of this work has been to

incorporate the viscous flow effects into the mathematical model that has been developed to predict the thermal stress

and strain produced in quenched plates of low-alloy steel. It was apparent from the review of previous work that the mathematical models developed to date have not made a realistic attempt to include the effect of stress

relaxation and creep on the stress generation process. Some of the models which attempted to include the viscous effect, did so in an arbitary fashion, without due

consideration of the type of steel used and the structural changes which occur during the quenching of steel. The mathematical model used in the present work, which was

( 99)

initially developed by Fletcher and Price did not

take viscous processes into consideration. The calculated results from this model showed a high level of agreement with the experimentally obtained results in a water

the case of an oil quench. This situation emphasised the need for modification to the mathematical model, and it was considered that the exclusion of viscous processes was a major defect in the model, and was responsible for

the lack of agreement with the experimental results in the case of an oil quench.

The approach taken in the present work has been to obtain experimental data of the stress relaxation process at temperatures between 850°C and 100°C and to represent them quantitatively by the use of mechanical models. It was found that the standard linear solid gave the best representation of the experimental behaviour of the type of steel used. The use of this model was further validated by a comparison of the experimental creep rates, obtained at high temperatures, with the creep rates predicted by this model. Isothermal viscous flow data may be criticised when it is applied to a continuous cooling situation.

This is because there may be a complex interaction between deformation processes at high temperatures and those that occur later in the quench, when the temperature is much lower. This aspect of the problem has not been considered experimentally during the present work, but the examination of several different models of the viscous process in the calculation have given an indication of the extent to which such effects might influence the thermal stress generation process. The method that gave the best agreement between the experimental and predicted

residual stresses and strains suggests that the prior • deformation of the material during the earlier stages

of the quench had little effect on subsequent viscous flow until a relatively low temperature (230°C) was reached. Below this temperature no viscous processes were present i.e. the small amounts of stress relaxation

suggested by the isothermal experiments were completely inhibited by the deformation that occurred earlier in the quench.

The various methods by which viscous processes had been introduced into the mathematical model were tested by an investigation of the martempering process, where a particularly long time is available for the viscous

processes to influence the generation of thermal stress and strain at intermediate temperatures. Again method 4,

which represents viscous flow as a time independent process at temperatures above 230°C, gives the best agreement with the experimentally determined stress and strain distributions. Although this agreement is very good in the case of stress, the level of discrepancy in the case of strain is about identical to that produced in a direct oil quench. Hence substantially different

thermal histories, with markedly different amounts of viscous flow led to virtually the same level of agreement with experiment. This would appear to indicate that the remaining descrepancy in the case of residual strains in slowly cooled material is not due to inaccuracies in the modelling of the viscous flow processes, but to some

other cause.

Experimental investigation of 835M30 steel have shown that the changes in length associated with the martensitic transformation are significantly affected by the magnitude and nature of the stress applied during the transformation process. The attempts to include this effect (transformation plasticity) into the mathematical model results in a further improvement in the level of

agreement between the predicted and experimental residual stresses in an oil quenched plate (see figures 112 and 120), but reduces the level of agreement in the case of water quenched material (figures 117 and 123). Furthermore

surprisingly good agreement is found when transformation plasticity is taken in conjunction with certain models of the viscous flow process which have not been very

successful when transformation plasticity has been ignored. However in all the cases considered a poorer level of

agreement is obtained between the predicted and experimental residual strain distributions. This is possibly due to

the method by which transformation plasticity, which is a heterogeneous effect, is introduced into the mathematical model, which' has been developed to consider only

homogeneous deformation. A better understanding of the heterogeneous nature of the transformation plasticity is required, along with modification of the mathematical model, in order for it to be capable of handling this

complex deformation phenomenon. The application of transformation plasticity to thermal stress generation

during quenching is a very recent development although at the present time it is the subject of intensive

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study ’ . It is to be expected that significant progress will be made in the near future.