M.T.& C. Engineers - Cryogenic 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 - CRYO TREATMENT

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?

Cryo treatment of metal involves controlled cooling below 0° C in order to alter the material microstructure, and enhance its strength and wear properties. Hardened alloy steel parts are particularly responsive to this treatment, as are carburised gears and shafts. Aluminium alloys can also be stress relieved by a process known as "up-hill quenching" from sub-zero temperatures, without the loss of strength normally associated with high temperature stress relieving processes.

Most alloyed steels are rarely fully hardened during normal heat treatment processes, leaving microstructural areas of untransformed austenite (a soft, low strength phase). Cold treatment of steels involves the controlled cooling of the metal to below the 100% martensite (a hard, high strength phase) transformation temperature. After a critical dwell time at this temperature, the metal is reheated slowly to above room temperature for a stress relief treatment. Simply ... the material structure is optimised to give a resultant structure that satisfies the metallurgical requirements of the alloy.

The whole cycle is customised by MT&CE to each individual part on the basis of material, and part configuration. The opposition treat all parts to the same cycle, sometimes with dire consequences. I have a racing pinion that was cryogenically treated in Melbourne that failed quickly by brittle fracture . . . the structure revealed brittle phases that could be attributed to poor treatment practice. It is easy to see why cryo has achieved a reputation for creating brittle parts when the metallurgy of the part is not considered by the cryo treater!

Stress relief of aluminium alloys involves cycling the part below and above room temperature. No microstructural changes are involved in the process. Stress relief is important for parts subjected to cyclic loading (eg. conrods and pistons), or exposed to corrosive environments such as marine atmospheres.

WHAT ARE THE BENEFITS?

Depending on the alloy composition and the prior hardening cycles, the benefits are increased strength, greater dimensional stability or microstructural stability, improved wear resistance, and relief of residual stress. Hardened steels also have fewer tendencies to develop grinding cracks.

Tool steels and carburised gears exhibit improved wear resistance increases between 18% and 560%, depending on the prior hardening cycles.

Careful dressing, cryo treatment and shot peening of gears from a high performance drag racing motorcycle gearbox allowed for an increase in output capability of 200% without pitting or wear of the teeth. The reduction in heat associated with deformation of the gear teeth also resulted in power gains to the rear wheel. In another case cryo treatment and shot peening of a Group A Touring Car differential set allowed the differential to operate at up to 190 degrees C (cooling pump failure) for over 5 hours without evidence of metal flow on the teeth, or cracking. Cryo treatment has also been used successfully in Group A and Group N rally car gearboxes, to extend the life of factory gearbox components. Even the humble Volkswagen, Ozzie 4 speeds, Commodore, Mitsubishi, Daihatsu, Subaru and ZF auto transmission (hot XR6 turbo cars) gearbox's have benefited from careful grinding and cryo treatment, followed by shot peening. Our customer have high praise for our services, and refer their associates without reservation. A fully treated Holden three speed used in Appendix J racing went from breakage in a meeting, to 5 years of successful racing with further engine modifications.

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!

Cast iron disc brake rotors and brake drums for race cars have responded well to cryo treatment, with customers reporting easier brake pad bedding periods, an increase in life before cracking of the working surfaces, and reduced distortion. Our personal experience on the F1 roadracing sidecar and Jedi Formula Libre has also supported these benefits. A Healey that regularly races at historic meetings had the drum brakes successfully cryo treated, and the drums ran without warping that normally resulted in pulsing brakes.

The Mazda rotary engine brigade have benefited from cryo treatment of all cast iron rotor seals, and the stationary / rotor gears. Experience has shown that the engines can be tuned to higher power outputs without premature failure of the otherwise brittle cast iron apex seals, and dressed/cryo/peened gears last longer before cracking.

Valve train components that benefit from cryo treatment includes valves, lifters, rockers (especially those Cortina OHC), and retainers.

Uphill quenching of aluminium alloys can result in a reduction of residual stresses in the metal of between 25% and 83% depending on the alloy and the temper condition. The reduction of residual stresses in the material can increase the load bearing capacity of the part. The process does not not strengthen heat treatable aluminium alloys that have been over-aged or inadequately heat treated.

Contact Stephen Hooker on 0419 498 115 for your cryo treatment and shot peening requirements today.

WHAT WON'T IT DO?

I felt I had to add to this . . . negative feedback never comes back direct to us, and is usually spread quickly by hearsay. The cryo process is NOT a miracle cure-all (although other service providers may tell you otherwise) and will not correct the following conditions:

Case hardened steels with massive carbides or ferrite in the structure. These phases are a result of poor initial heat treatment, and result in a dramatic reduction in strength. If this is the condition of your part, then it is destined to fail anyway! In addition, to answer your next question . . . it cannot be readily determined if the part has these defects without partly destroying it.

Alloy steels that have been over-tempered (low hardness) cannot be improved without rehardening and tempering to the correct hardness. This is a metallurgical fact! And, if the material is being loaded enough to break within a few load applications your part needs to be redesigned with a higher strength material and larger sections before you look at cryo. Cryo is an OPTIMISATION process that will improve fatigue life and wear properties, and produce smaller gains in bulk strength / hardness. And some mechanical sympathy would help . . . shock loads on any part will reduce its life dramatically . . . it may be wise to review how the loads are applied to the part if you expect it to live longer . . . cheap insurance really!

Materials that have been overheated or overloaded are also destined to fail, and the cryo process will not help them miraculously come back to life either. Bent or deformed parts are beyond stress relief . . . bin them!

If there is any doubt about the effectiveness of the treatment call Stephen. He regularly provides metallurgical backup support to customers to help them solve their material related problems.

The last words . . . there are a number of companies out there with no metallurgical knowledge, a tank of nitrogen and a one-process-does-everything attitude. They can actually do more harm than good to your parts, and I have a differential pinion with less than a race on it here to prove it. One of these companies made a pinion gear that normally lasts three endurance races hard and brittle . . . you have been warned!

This page last updated: 16/03/24