How cool is that!
From its roots in Scotland, the cryogenic treatment of metals and plastics was
developed by NASA and subsequently commercialised for many applications.
The greatest cost savings have arisen when the process is applied to cutting tools.
HEATtreatment implies the use of elevated
temperatures, but the cryogenic process uses the belowzero
range as well. This offers significant benefits to
cutting tool users. Typical increases in tool life may be
around 300% for drills, and 500% for a carbide insert
cutting titanium.
Andy Priscott, managing director of Cryogenic
Treatment Services, explains how the process works:
‘Using sophisticated cryogenic chambers that are
computer controlled we can model a cooling and reheating
curve down to -195˚C and up to 300˚C. The key to
the process is the tight control of the temperature curve.
Each process requires a different curve. Some remain at -
195˚C for a number of hours and are slowly brought back
to room temperature; some materials require re-heating to
temper the material after cryogenic hardening.
‘In the past scientists discovered that immersing some
metals in liquid nitrogen could increase their wear
resistance, particularly in aircraft engines, giving a longer
in-service life. But there were disadvantages such as overly
brittle material. Thanks to NASA, developments in
computer modelling and thermal insulation of the
chamber, we are now able to use gaseous nitrogen. The
tightly controlled computer process allows us to produce
significant improvements to the whole structure of the
materials being treated. In particular, we can demonstrate
extended life of cutting tools that significantly reduce
replacement and set-up costs for manufacturers.’
‘Cryogenics is a
mixture of physics and chemistry. In ferrous steels, it’s all
about the transformation of austenite, a large soft crystal,
into martensite, a smaller, harder, more compact crystal.
And when we start to get down to -185˚C N-carbides start
to grow throughout the structure. The net result is that the
crystal structure is transformed with the boundary
adhesion between the various crystal elements also
improved - both delivering better wear properties. We can
actually hold temperatures as low as -195˚C (hence the
company’s web and email address are 195below.co.uk).
‘Slowly cooling a tool steel to deep cryogenic
temperatures and soaking it at this low temperature for a
number of hours changes the material’s microstructure.
Almost all of the austenite retained in the steel after heattreating
is transformed into a harder form, martensite, by
the deep cryogenic process. An additional result of a deep
cryogenic “soak” is the formation of fine carbide particles,
called binders, to complement the larger carbide particles
present before cryogenic treatment. This, however,
depends on the alloying elements used, such as chromium
carbide or tungsten carbide, in the steel.
‘After treatment, the cutting tool will have better wear
properties. The increase in resistance to wear occurs with
the ability of the tool material to defend against a particle
penetrating or gouging its surface. The martensite and fine
carbide formed by deep cryogenic treatment work
together to reduce abrasive wear. The fine carbide particles
support the martensite matrix, making abrasions and
scuffing of the cutting tool less likely during a cutting
operation. When a hard particle or foreign body is pressed
onto the tool’s surface, the carbides resist wear by
preventing the particle from ploughing into the surface.
‘Some of these benefits may be achieved through
standard tempering that also transforms austenite into
martensite. However, standard tempering may not bring
about a complete transformation in some tool steels.
Cryogenic treatment improves tool performance by
transforming more austenite into martensite. Overall, if an
alloy contains austenite, and this austenite responds in
some degree to heat treatment, further improvements will
be seen after deep cryogenic tempering.’
‘Cryogenic processing of tooling is not a replacement to
traditional heat treatment and coating techniques,’ stresses
Priscott. ‘It should be seen as a complement, an extension of
heat treatment.’ To encourage companies to experiment, the
process is priced to be attractive - and the company offers to
refund the cost of cryogenic processing if production records
clearly indicate that the processed tools show no cost
advantage over the unprocessed tools. MWP