M.T. & C. Engineers - Shot Peening Info Sheet

Metallurgical Testing & Consulting Engineers

Silverdale. NSW. 2752. Australia

(Near Warragamba Dam -Covering Sydney Metro)

Phone: (02) 4774 1017

Mobile (Stephen Hooker): 0419 498 115

INFO SHEET - SHOT PEENING

Metallurgical Testing and Consulting Engineers provide a non-destructive testing and advisory service to the high performance motor-racing industry. We also provide an in-house parts improvement capability including controlled shot peening and cryogenic treatment. Our prompt and professional service ensures that our Customers can operate their business operations at a high level of confidence and reliability.

WHAT IS IT?

Shot peening is the process of cold forming the surface of a part by means of the impacts off a propelled stream of round hardened steel shot particles. The result of this process is a uniformly dimpled surface, the roughness being determined by the shot size, peening intensity and the hardness of the part being peened. The process is non-abrasive.

The shot peening process is used to improve the fatigue properties of the part by the introduction of compression forces in the surface layer. The presence of this surface compressive force serves to retard the initiation and growth of fatigue cracks.

Fatigue failures are easily recognised, and usually start from a focal point on the surface. These focal points are stress raisers such as fillets, holes, keyways, seams, laps, tool marks, scratches or structure variations. When fatigue failures are encountered, these stress raisers should be eliminated where possible.

The size of the shot used by MT& C Engineers is related to the size of the smallest fillets / openings that the shot must penetrate, and the peening intensity required. The peening intensity is monitored regularly by the use and measurement of Almen test strips (many of the opposition would not know what these are!). The peening intensity is normally selected from military specification S-13165A, based on the material strength and the section thickness. The recommendations are based on the lowest peening intensity capable of producing the desired surface compressive stress. The intensity may be considered excessive in thin parts if the tensile forces in the core material outweigh the beneficial compressive forces induced at the surface.

The process should not be confused with shot blasting which is a cleaning process using broken shot as an abrasive. The surface finish resulting from shot blasting is coarse, contains microscopic notches, and can result in a reduction in the fatigue performance of the part. Many of our converts have remarked on the improvement in surface finish resulting from switching to our services, Few "blasting " companies are aware of the precise controls over the peening process that are needed to ensure optimum fatigue performance of the shot peened part. I recall seeing a set of H-beam rods "blasted" by a local company that revealed 1mm warps in the webs . . . the owner had to discard them as they could not be recovered.

WHAT ARE THE BENEFITS?

Reported increases in the life of springs are in the order of 400% to 1200%, depending on the extent of peening stress already imparted on the spring. Past experience with shot peening of valve springs from a vintage engine resulted in an increase in the installed spring pressure, and an extension of life of an otherwise irreplaceable part.

Shot peening increases the fatigue life of gears, and case histories have revealed over 500% increase in life. Other tests on drive pinions give 40% to 414% life increases with peening. In many instances satisfactory transmission or rear axle gears are shot peened, and then the loads increased under racing conditions. For a greater improvement the gears can be dressed and cryo treated by us prior to shot peening to maximise their strength (especially those Ozzie 4 speeds).

MT&CE have recently undertaken an extensive testing program on the effects of the cryo and peening processes on molybdenum metal spray coated synchro's from Mini and Healey gearboxes. A range of gears were checked dimensionally, visually, and torque tested (baulk ring / synchro friction load) at various stages of the full treatment regime, including dressing, cryo and shot peening. The torque tests were also expanded to include different lubricants, including friction modified oils and greases. On conclusion of testing it was determined that here are no adverse effects on the molybdenum metal spray coatings provided that they are of serviceable dimensions, and have not been used with friction modified oils. Customers are advised to ensure that the gears are servicable before getting them treated, and MT&CE reserve the right to not cryo treat or shot peen any parts that are deemed to be sub-standard. Interested customers can contact Stephen for detailed information on the report. PS: We have experienced some recoated synchro's that appear to have been coated with molybdenum disulphide that washes off with petroleum solvent . . . if you have some of these we suggest you try the solvent clean test yourself, and then send them back to your supplier if it washes off . . . the standard metal spray definitely does not wash of in solvents!

Shot peening of the fillet radius areas of crankshaft journal increases the fatigue life dramatically, and in many cases, customers have deleted nitriding in favour of shot peening the fillets. Our experience with nitrided crankshafts that have not been manufactured from nitriding grade steel is that they are prone to brittle fracture when flexed inside the engine. Further, it has been observed that nitrided cranks that have suffered from oil starvation are more prone to journal overheating cracks. This is in contrast to the belief that these journals can be easily cleaned up after a bearing failure and put straight back into racing service! An added bonus of shot peening over nitriding is the absence of distortion or growth of the crankshaft.

Other parts that are shot peened include valves (in collet grooves), conrods, axles, stub axles, torsion bars, marine propellers, skegs and rudders, and exhaust megaphones. All have benefited from increased fatigue life, or an increase in service loads. As a matter of fact you will find that all high quality aftermarket racing components are shot peened to improve their fatigue performance ... makes sense when you consider how close to the fatigue strength limit these parts operate.

After-market conrods (Carillo, Falicon, Oliver etc) that have been overheated on the big-end without lowering the material hardness (we can check the hardness for you), are recommended to be crack tested, low-temperature stress relieved (another one of our services), and then re-peened to restore the beneficial effects of the original shot peened surfaces. In this way an expensive part, which may have been superficially damaged by oil starvation, can be salvaged and restored to racing service.

Previously peened parts that exhibit damage that has been dressed back (eg. conrods dressed after a lifter failure) can be re-peened to restore their original fatigue resistance.

The peening process also has applications in forming hardened materials, and straightening bent parts. I have even peened straightened the Mira clamping plate to reduce the bow to make it more suitable for use on aluminium motorcycle heads.

After peening the material should not be heat-treated, machined or straightened, as the effectiveness of the peening process will be reduced. This information is provided with the job card accompanying the shot peened parts.

Contact Stephen Hooker on 0419 498 115 for your shot peening requirements today. We can also pick-up and deliver in Western Sydney.

This page last updated: 03/02/17