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Research Papers: Materials and Fabrication

Post-Autofrettage Thermal Treatment and Its Effect on Reyielding of High Strength Pressure Vessel Steels

[+] Author and Article Information
E. Troiano, J. H. Underwood, C. Mossey

 US Army RDT&E Center, Watervliet, NY 12189-4050

A. P. Parker

 Cranfield University, Swindon SN6 8LA, UK

J. Pressure Vessel Technol 132(6), 061402 (Oct 15, 2010) (5 pages) doi:10.1115/1.4001209 History: Received August 05, 2009; Revised November 24, 2009; Published October 15, 2010; Online October 15, 2010

The autofrettage process of a thick walled pressure vessel involves applying tensile plastic strain at the bore of the vessel, which reverses during unloading and results in favorable compressive residual stresses at the bore and prolongs the fatigue life of the component. In thick walled pressure vessels this process can be accomplished with either a hydraulic or mechanical overloading process. The Bauschinger effect, which is observed in many of the materials used in thick walled pressure vessels, is a phenomenon, which results in lower compressive residual stresses than those predicted with classic ideal isotropic hardening. The phenomenon is a strong function of the amount of prior tensile plastic strain. A novel idea, which involves a multiple autofrettage processes, has been proposed by the present authors. This process requires a low temperature post-autofrettage thermal treatment, which effectively returns the material to its original yield conditions with minimal effect on its residual stress state. Details of this low temperature thermal treatment are proprietary. A subsequent second autofrettage process generates a significantly lower amount of plastic strain during the tensile reloading and results in higher compressive residual stresses. This paper reports the details of the exploratory tests involving tensile and compressive loading of a test coupon, followed by a low temperature post-plastic straining thermal treatment, and subsequent reloading in tension and compression. Finally results of a full scale safe maximum pressure (SMP) test of pressure vessels are presented; these tests indicate a significant increase (11%) in SMP.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic depicting the stress versus the strain history of the first and second autofrettage

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Figure 2

Initial stress versus the strain applied to the A723 tensile test specimens

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Figure 3

Second loading of A723 specimens with and without thermal treatment

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Figure 4

Second loading of HB7 specimens with and without thermal treatment

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Figure 5

Second reverse loading of A723 specimens with and without thermal treatment

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Figure 6

Second reverse loading of HB7 specimens with and without thermal treatment

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Figure 7

SMP of A723 steel stub tubes (1055 MPa) subjected to one autofrettage cycle (solid line) and two autofrettage cycles (dotted line)

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