Abstract
Internal Quenching is an innovative heat treatment method for difficult to access component sections. Especially, the microstructure, as well as the residual stress state at inner surfaces, of thick-walled tubes can be adjusted with the presented flexible heat treatment process. Based on multiphysical FE-models of two different steels, a simulative optimization study, considering different internal quenching strategies, was performed in order to find the optimal cooling conditions. The focus hereby was on the adjustment of a martensitic inner surface with high compressive residual stresses. The simulatively determined optimal cooling strategies were carried out experimentally and analyzed. A good agreement of the resulting hardness and residual stresses was achieved, validating the presented Fe-model of the Internal Quenching process. The shown results also indicate that the arising inner surface state is very sensitive to the transformation behavior of the used steel. Furthermore, the presented study shows that a preliminary simulative consideration of the heat treatment process helps to evaluate significant effects, reducing the experimental effort and time.
Highlights
Compressive residual stresses are induced at inner surfaces by plastification, enhancing the fatigue life
A heat treatment resulting in high compressive residual stresses at the inner surface was experimentally carried out
In the case of AISI 1045, the inner surface was quenched with a water pressure of 60 bar which corresponds to a heat transfer coefficient of around 30 kW/mm2
Summary
The use of high-strength steels, as well as precision machining, is necessary to ensure a reliable performance of pressurized components. Regarding conventional surface heat treatment methods, like case hardening, induction hardening, or laser hardening, surfaces are strengthened by inducing compressive stress in the surface. By using a high pressure water flow during quenching, heat transfer coefficients over 20.000 W/m2 K can be reached, enabling beneficial heat treatment methods, increasing/inducing compressive residual stress at the surface [11,12]. The Internal Quenching heat treatment method bases on induction heating and a local intensive water quenching process at the inner surface of thick-walled tubes. The effort of the mechanical finishing is reduced or the fatigue resistance of high pressure steel components can be increased as it is shown in [13], where the development and the influence on fatigue properties of residual stresses induced by heat treating is described
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