Manual On Shot Peening

Author:  SAE
Source:  SAE: SP- 84
Doc ID:  1984044
Year of Publication:  1984
Abstract:  
This manual on Shot Peening is intended to be a practical aid to engineers, designers, and men in the shop, pointing out both some of the possibilities and some of the limitations of the process. It has been prepared by a group of men of broad experience with the process and whose leadership in the field is acknowledged. Shot peening may be defined as the process of cold working the surface of a structural or machine part, by means of a driven stream of hard shot. The purpose of the process is to improve the fatigue properties of the shot-peened part. While all factors which affect the improvement are not completely understood, it is generally attributed to the introduction of compressive stresses in the surface layer and to the slight hardening effect caused by the peening action of the shot. It is said that most fatigue failures occur only in tension, so the compressive stress in the shot-peened object is in effect a pre-loading device, opposite in direction to the stress applied in service. Thus, the actual service stresses are reduced to a safe level by the counter-effect of the pre-loading induced by shot peening. The principle of improving strength of metals by cold working undoubtedly was discovered early in civilization, as ancient man hammered out his tools and weapons. In more recent times, the blacksmith hammered the tension side of buggy springs because he knew it made them better. However, it was not until 1927 that the glimmerings in the mind of man regarding this process were voiced by Herbert in his paper, "Work Hardening of Steel by Abrasion". At a later date, definite claims were made in another paper, "Cloudburst Process for Hardness Testing and Hardening". None of this information was very definite, but it was sufficient to arouse interest and to start additional investigations, largely in the spring industry. These investigations and their results were summed up in a paper given at the American Society for Metals 1940 Symposium on Surface Treatment of Metals entitled, "Shot Blasting and Its Effect on Fatigue Life". This encouraged the publication of many articles from numerous laboratories and these added much to the pool of knowledge. Shot peening is not the only method by which these beneficial surface stresses may be introduced into a part. The most important of the other methods are as follows: Cold working by rolling, by stretching, by compressing, by bending and by twisting. Heat treating, and in particular, surface treatments by induction and flame treating. Changing the chemical composition of the surface layer by carburizing, nitriding, etc. Shot peening possesses several advantages over any of these methods, such as flexibility, control of stress intensity, safety from the point of view of not introducing unwanted or harmful stresses by accident and, in many instances, economy.