Electronics Incorporated - The Shot Peener - Shot Peening Library
Title: Influence Of Shot Peening On The Microstructure And The Bending Fatigue Strength Of Bainitic-austenitic Nodular Cast Iron
Author: Ebenau, Lohe, Vohringer, Macherauch
Source: Conf Proc: ICSP-4, (p.389-398)
Publication year 1990
Document number: 1990038
Number of pages: 10
Abstract:
Bainitic-austenitic nodular cast iron, which is produced by austenitizing and incomplete isothermal transformation, shows high tensile strength combined with remarkable
ductility. In the present paper, the consequences of shot peening on the surface and subsurface material states and on the bending fatigue behaviour are investigated.
Pearlitic nodular cast iron was austempered in a salt bath at 300+°C(type A) and 380+°C(type B). Type A contains about 22 vol.-% retained austenite and type B about 39
vol.-%. By shot peening, characteristic depth distributions of residual stresses, half width of X-ray interference lines, retained austenite and microhardness are produced.
In both material states, a strong peening induced transformation of retained austenite into martensite appears. Therefore the depth distributions of compressive residual
stresses show two maxima: the first below the surface and the second directly at the surface. This phenomenon can be explained by the volume increase as a consequence of
peening induced transformation of retained austenite into martensite.
T
he increase of fatigue life and fatigue limit by shot peening is much
more significant for type B (115 %) than for type A (31 %). In both cases, the crack initiation site is shifted below the surface. The results obtained cannot be
understood on the basis of the local fatigue strength concept alone, which takes into consideration of the peening induced alterations of residual stresses and hardness. It
is assumed that the high fatigue strength of shot peened material of type B is caused by a much higher resistance against crack initiation below the surface than at the
surface. The underlying mechanism is a strong workhardening and strain assisted transformation of retained austenite into martensite, which locally produces large
hydrostatic stress components retarding the spreading and the coalescence of microcracks present in the investigated material states already after the first cycle.
Descriptors: Nodular cast iron; Austempering; Bainite; Retained austenite; Bending fatigue strength; Fatigue life; Fatigue limit; Transformation of retained austenite into
martensite; Strain assisted transformation; Decohesions; Microcrack nucleation; Crack propagation
Download full text pdf file
www.shotpeener.com
This page last revised: 04/24/2008