Found the discussion on cryo treating:
http://pforums.company-hosting.com/f...php?t=67138159
Two good posts:
http://pforums.company-hosting.com/f...php?t=67138159
Two good posts:
As a metallurgist (23 years), I found most of the above interesting and, at times, amusing (sorry guys )
Cryogenic treatment has been around in the aircraft industry for decades - and it does work, but would be of debatable value for anything but (very) high performance car parts. The easist way I can explain it is this:
Keeping a compex subject simple...when steel is heat-treated it is usual to soak above the austenite phase transition temperature. Subsequent quenching 'transforms' the austenite to martensite. The steel is then usually tempered to the appropriate hardness/toughness. BUT, because we don't live in an ideal world, at the time of quenching, all of the austenite is invariably not 'transformed' to martensite - there is always residual austenite, the amount will vary dependant on a HUGE range of factors.
This small amount of residual austenite contributes little to the strength of the steel (relative to the martensite), so is 'undesirable', if you will. However it is also unstable or 'metastable', since it 'wants' to 'transform' martensite. All that is required is something to 'push it over the edge' as it were, and this is where cryogenic treatment comes in - by lowering the temperature of the steel sufficiently, the residual austenite is transformed and the mechanical properties of the steel is improved. [and, yes, once transformed it stays transformed even when the steel returns to room temp]
[as an aside I would note that 'martensite' is actually a metallographic artefact - it has exactly the same crystallographic form as austenite, but 'distorted' due to the presence of carbon]
'Toughness' is the resistance of a material to crack growth, by transforming residual austenite to martensite the mechanical properties of the steel are improved and consequently so should the toughness (but that's a HUGE generalisation).
The subject of heat-treatable aluminium alloys is a whole different ball game.
Cryogenic treatment has been around in the aircraft industry for decades - and it does work, but would be of debatable value for anything but (very) high performance car parts. The easist way I can explain it is this:
Keeping a compex subject simple...when steel is heat-treated it is usual to soak above the austenite phase transition temperature. Subsequent quenching 'transforms' the austenite to martensite. The steel is then usually tempered to the appropriate hardness/toughness. BUT, because we don't live in an ideal world, at the time of quenching, all of the austenite is invariably not 'transformed' to martensite - there is always residual austenite, the amount will vary dependant on a HUGE range of factors.
This small amount of residual austenite contributes little to the strength of the steel (relative to the martensite), so is 'undesirable', if you will. However it is also unstable or 'metastable', since it 'wants' to 'transform' martensite. All that is required is something to 'push it over the edge' as it were, and this is where cryogenic treatment comes in - by lowering the temperature of the steel sufficiently, the residual austenite is transformed and the mechanical properties of the steel is improved. [and, yes, once transformed it stays transformed even when the steel returns to room temp]
[as an aside I would note that 'martensite' is actually a metallographic artefact - it has exactly the same crystallographic form as austenite, but 'distorted' due to the presence of carbon]
'Toughness' is the resistance of a material to crack growth, by transforming residual austenite to martensite the mechanical properties of the steel are improved and consequently so should the toughness (but that's a HUGE generalisation).
The subject of heat-treatable aluminium alloys is a whole different ball game.
Ideally you would want to quench from the austenite phase to the lowest temp possible to acheive full transformation, but, in practice, this is impractical because it would invariably lead to cracking of the component (esp to LN temps: -196C or thereabouts). Usually a compromise is acheived in consideration of the particular steels TTT diagram (Time-Temperature-Transformation), it's thickness, geometry, etc.
As-quenched martensite is very brittle and hard, and steels are rarely used in that form. Subsequent tempering produces a more acceptable structure - but still containing some austenite. Cryogenic treatment 'transforms' the remaining austenite to 'hard' martensite which is more beneficial than leaving it as austenite - plus what is 'transformed' remains contstrained by the surrounding structure which actually has beneficial effects in terms of residual stresses, etc.
Cryogenic treatment is invariably applied to the final product, but usually before final machining, esp in high precision components (read: aircraft parts) because minor dimensional changes can occur because of the process.
I would be highly sceptical about massive gains in material props because of this treatment alone, esp with car parts. OTOH, weight/performance gains are critical in aircraft parts, particularly military - and this is where the process has been used very successfully.
The bottom line: for a road going performance car - forget it, but OTOH I'm sure the F1 people are using it
As-quenched martensite is very brittle and hard, and steels are rarely used in that form. Subsequent tempering produces a more acceptable structure - but still containing some austenite. Cryogenic treatment 'transforms' the remaining austenite to 'hard' martensite which is more beneficial than leaving it as austenite - plus what is 'transformed' remains contstrained by the surrounding structure which actually has beneficial effects in terms of residual stresses, etc.
Cryogenic treatment is invariably applied to the final product, but usually before final machining, esp in high precision components (read: aircraft parts) because minor dimensional changes can occur because of the process.
I would be highly sceptical about massive gains in material props because of this treatment alone, esp with car parts. OTOH, weight/performance gains are critical in aircraft parts, particularly military - and this is where the process has been used very successfully.
The bottom line: for a road going performance car - forget it, but OTOH I'm sure the F1 people are using it
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