E-Archive
Articles
in Vol. 26 - January Issue - Year 2025
Shot peening – History and Development

Image Analysis for Shape and Size (Model Provided by CSEE, Purdue University)

Ervin Rotary Atomized Powder

Gas Atomized Powder
Background
The shot peening industry is constantly evolving, and this was apparent in various presentations at the recent shot peening workshop in Colorado Springs, Colorado (USA). Developments discussed involved controls, secondary processes and overall improvements to everything we do in our daily work. Growth is best explained by comparisons, as in ‘there are more users of this process now than in early 2000s’, ‘equipment repeatability is better than a decade ago’ and so on. James Kernan, an industry colleague who has spent his career promoting best practices in shot peening in the US Army is now a trainer and NADCAP auditor. At this workshop, he introduced the history of shot peening to the audience and his topic made this presentation richer.
The earliest recognition of the beneficial effects of cold working on increasing fatigue resistance was documented by a German named O. Foppl in his article ‘Compression of the Surface of Steel Machine Parts’ in 1929. Later, another related paper presented by J.H. Frye and G.L. Kehl to the American Society of Metals in 1937 titled, ‘The Fatigue Resistance of Steel as Affected by Some Cleaning Methods’ described the improvements in fatigue life after shot blasting (the process was not explicitly recognised as shot peening).
It was in 1942 that John Otto Almen patented what we now know as the Almen Gauge, which allowed the measurement of arc heights and subsequent calculation of the process intensity. Shot peeners today are familiar with SAE J443. This recommended practice describes the procedure for plotting a saturation curve, an industry-accepted practice for validating peening processes and calculating intensity. Interestingly, a historic version of SAE J443 in 1952 explained intensity as “the gage reading corresponding with the point A where the curve flattens out is generally taken as the measurement of the intensity of that particular peening. In some cases, this point is difficult to pick out and requires judgment”. The point A mentioned referred to the T (or T1) time on the saturation curve. It was not until 1984 that the 10% rule we are familiar with was adopted to define saturation, more precise language following in later years. It is important to acknowledge the initiation of this process so that we can be sure that we are heading in the right direction in the future.
Aspects of growth
When it comes to growth prospects, several areas present themselves as ready candidates. Process repeatability is the key in shot peening and it is logical to grow in this direction in order to guarantee consistency and repeatability in peening results. This is true for a single part (Aerospace) or for each individual part belonging to a batch of several hundreds (Automotive) being peened.
Peening media
The first input that comes to mind is the shot, both cast and cold drawn (cut wire). Steel shot manufacturers have improved their processes over the years to maintain their product consistency. Michael Konecny is the Technical Services Manager at Ervin Industries, responsible for providing quality guidance to all three Ervin plants in North America and two in the EU. “We are in a constant state of process control and innovation to ensure that the key elements of Durability and Transmitted Energy are assured with our blast cleaning and shot peening media. This involves paying close attention to the chemistry and heat treatment during manufacturing, which in turn maintains particle metallurgy, hardness and microstructure within tight tolerance to limit the occurrence of mechanical imperfections such as cracks, voids and shrinkage”, Michael explained. To put this in perspective, the earliest version of MIL-S-13165 in 1953 listed cast iron among the material types for peening media. Cast iron is not used for peening (or cleaning in present times) due to its brittle nature and propensity to break into sharp-edged particles. Sharp edges, though beneficial in certain cleaning applications, is detrimental to the part being shot peened.
Shota Watanabe is the President at Toyo Seiko North America, a quality cut wire manufacturer that also serves the shot peening industry. Shota summed up their quality improvements over the years as, “At Toyo-Seiko, we have perfected our heat treatment process to transform the microstructure of our cut wire from brittle fibre pearlite to more durable martensite that enables longer life cycles than material that has not been heat treated. This manufacturing technique ensures that two important aspects of conditioned cut wire media are maintained - (a) processing fully rounded particles and (b) relieving the tensile stress and replacing with favourable compression in the conditioned particles”.
Shot velocity
Blast cleaning and shot peening are defined by the quantum of impact energy, of which velocity is a key factor. Though saturation curves validate the process, monitoring and maintaining consistent velocity keep the process in check. Indirect estimation of velocity is done through air pressure in an airblast machine and wheel diameter and speed during a wheelblast process. Since velocity has a direct relationship with the intensity and compressive residual stress, velocity measurement devices offered by several manufacturers (Progressive Surface, Wheelabrator, Oseir, etc.) allow real-time measurement of velocity as the peening media discharges from the blast nozzle. By checking media quality (more on this later) and monitoring media velocity as it is generated, consistent peening results can be obtained.
Media maintenance
A group of graduate and undergraduate students at Purdue University (Center for Surface Engineering and Enhancement) have been working under the guidance of Prof. Paul Mort to characterise media particles through image analysis. Their focus has been on studying shot samples (samples containing about 10,000 particles) using Aspect Ratio (ratio of width to height), Form Factor and Elliptical Form Factor (a shape descriptor to represent the cylindrical shape of a shot particle). They have conducted their study using a device called SolidSizer (Canty Process Technology) that uses a vibratory feeder to monitor the flow of media particles and a high-speed camera to capture images (10K to 100K images). The purpose of this exercise has been to offer a more accurate and quantifiable assessment of the shape and size of shot (cast and cut wire shot) particles than currently used methods.
Shot size differentiation with multiple sieves and shape analysed visually through magnification remain factors in determining whether a media sample is acceptable or should be rejected. Most would agree that this process is time-consuming, lacks repeatability and is prone to human error in multiple stages. Researched and improved image analysis is a giant leap in media quality assessment.
Other media developments
Since most aerospace shot peeners are also involved in other processes such as thermal spray involving a different type of media, it is useful to discuss complementary developments in this field. Mark Hash, the Research & Development Director of Ervin Technologies, Tecumseh, Michigan explains, “Ervin Technologies uses a unique centrifugal or rotary atomisation technology to manufacture metal powders. Powders manufactured tend to be more spherical than those produced by gas atomisation. This degree of sphericity enables superior, uniform flow and packing, making our powders ideal for all types of powder metallurgy, including manual torch powder welding, thermal spraying, cored wires, additive manufacturing methods, and hot isostatic pressing. Ervin’s metal powders are also exceptionally clean with significantly lower oxygen contents than those produced by gas atomisation. These attributes plus uniform chemistries and consistent particle size distributions enable high-quality products of all types. Alloys currently available include Types 316L, 17-4PH, 304L, and a high-carbon Type 303”.
Future growth opportunities
The growth initiatives above are either ongoing or already in place. As in any developing process such as shot peening, there are endless possibilities for improvement. Encouraging more users in industries such as oil & gas, mining and medical implants to increase their adoption of shot peening will in turn lead to equipment and component manufacturers to innovate technology and focus their R&D efforts to meet the specific requirements of new applications (in addition to the traditional aerospace and automotive industries). Listed below are some of those possibilities:
- Real-time (and faster) measurement of compressive residual stress. Extending component service life through shot peening is the ultimate goal of the process. This is depicted by the compressive residual stress value expected by the component’s designer. Since this is not easily measurable immediately after peening, we develop the process by plotting saturation curves and periodically verify it with another Almen strip measurement. Real-time measurement, on the other hand, offers the ability to provide positive process validation as soon as the component is peened, potentially for every component being processed. This technology is currently available and being implemented in specific applications. The goal should be to improve its availability and acceptability as a valid measurement technique.
- Machine predictability – with the infusion of Artificial Intelligence in all aspects of manufacturing, it is important that our industry also reviews its applicability and makes use of it. Some areas where we could benefit from AI include:
- redictability of peening results (intensity and coverage) based on process parameter selection. A comprehensive database of material properties and their impact on coverage will help estimate cycle time (coverage rate) with relative accuracy rather than leaving it to guesswork and past experience. The correlation of these data with different intensity values will benefit both designers and operators. Considering that AI has made progress in other industries, it holds tremendous promise for our industry as well.
- redictability of maintenance - Shot peening results are negatively impacted by the wear of critical components such as the nozzle, classifier screen and flow control valves. Consider a scenario where declining nozzle life, the possibility of a hole in a classifier screen or a clogged flow control valve can be predicted 50 hours before the occurrence so as to allow preventive maintenance to be performed and reduce the likelihood of machine downtime or worse, an incorrectly peened part.
Summary
History has brought us to a stage where the benefits of peening are widely publicised, and carefully drafted specifications have demonstrated to provide repeatable results when conformed to. It is time to move the agenda forward to enhance technology through some of the initiatives identified in our discussion. There still exists possibilities in advanced material technologies to minimise wear inside the cabinet, reduce source noise and minimise leakage of media - in other words, reduce the annoyance factors of using a blast cleaning or shot peening machine! I look forward to reporting on these and other advancements in future discussions.
For Information:
Kumar Balan
Applications Engineer, Ervin Industries
Tel. +1.905.483 6890
