The History of Cryogenics
Cryogenics, as we recognise it today, started in the late 1800's when Sir James Dewar (1842 – 1923) perfected a technique for compressing and storage of gases from the atmosphere into liquids. (Some credit a couple of Belgians as being first to separate and liquefy gasses but being British we’ll stay with Sir James Dewar for now). These compressed gases were super cold and any metal that came in contact with the ultra low temperatures showed some interesting changes in their characteristics.
The first liquefied hydrogen by Sir James Dewar was in 1898 and a year later he managed to solidify hydrogen – just think on that for a moment… This is before electricity was common in houses, cars and buses a rare find and photography a rich mans hobby. By pure persistence and fantastic mental ability a whole generation of ‘Gentleman Scientists’ managed to bring into existence many things we both rely on and take for granted today.
Sir James Dewar managed to study, and lay the corner stones for the production of a wide range of gases that we use in our everyday lives, mostly without even realising it. He also invented the Thermos flask (how else was he to save his liquid gas samples), the industrial version of which still uses his name - ‘Dewar’.
Before we leave Sir James Dewar, his achievements deserve a mention...
With Sir Frederick Abel, he invented ‘smokeless gunpowder’ or Cordite (1889).
He also Discovered the Formula for Benzene (1867).
Back in the 1940’s scientists discovered that by immersing some metals in liquid nitrogen they could increase the ware resistance of motor parts, particularly in aircraft engines, giving a longer in service life. At the time this was little more than dipping a part into a flask of liquid nitrogen, leaving there for an hour or two and then letting it return to room temperature. They managed to get the hardness they wanted but parts became brittle. As some benefits could be found in this crude method, further research into the process was conducted. The applications at this stage were mostly military.
NASA led the way and perfected a method to gain the best results, consistently, for a whole range of metals. The performance increase in parts was significant but so was the cost of performing the process.
Work continued over the years to perfect the process, insulation materials improved, the method of moving the gas around the process developed and most importantly the ability to tightly control the rate of temperature change.
Technology enabled scientists to look deeper into the very structure of metals and better understand what was happening to the atoms and how they bond with other carbons. They also started to better understand the role that temperature plays in the treatment of metals to effect the final characteristics.
As with most everything in our lives today, the microprocessor enabled a steady but continual reduction in size of the control equipment required as well as increasing the accuracy of that part of the process.
It is only since the mid 1990’s that the process has started to become a commercially viable treatment in terms of ‘cost of process Vs benefits in performance’
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
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
Study of Cryogenics
Published ISIJ International Vol. 34 (1994), No. 2, pp.205-210
Role of Eta-carbide Precipitation’s in the Wear Resistance Improvements of Fe-12Cr-Mo-V-1.4C Tool Steel by Cryogenic Treatment
Fanju MENG, Kohsuke TAGASHIRA, Ryo AZUMA and Hideaki SOHMA
Muroran Institute of Technology, Department of Mechanical Engineering, Mizumoto, Muroran, Hokkaido, 050 Japan. 1) Muroran Techno-Center, Higashimachi, Muroran, Hokkaido, 050Japan.
The wear resistance of an Fe-12.2wt%Cr-0.84wt%Mo-0.43wt%V-1.44wt%C alloy tool steel after cold treatment at 223 K (-60f) and after cryogenic treatment at 93 K (-292f) has been investigated. The wear resistance of steels after cryogenic treatment is superior to that after cold treatment. The effects of cryogenic treatment on the microstructure were also studied by means of X-ray diffraction and transmission electron microscopy methods. Unlike cold treatment, cryogenic treatment improves the preferential precipitation of fine n-carbides instead of e-carbides. These fine carbide particles enhance the strength and toughness of the martensite matrix and then increase the wear resistance. The formation mechanism of fine n-carbide is discussed.
KEY WORDS: alloy tool steel; wear resistance; subzero treatment; cryogenic treatment; precipitation; n-carbide; retained austenite.
Cold treatment (subzero treatment), as an indispensable part of heat treatment of alloy tool steels, offered significant increases in the wear resistance. It is widely accepted that a major factor contributing towards its success is the removal of retained austenite. Conventional cold treatment has been carried out at higher than 173K. This temperature is
believed to be sufficient to fully transform any retained austenite to martensite in the quenched microstructure. However, more recent evidence has shown that wear resistance is further enhanced by cryogenic treatment at ultra low temperature (ultra subzero treatment), such as liquid nitrogen temperature. Despite the numerous practical successes of cryogenic treatment and research projects undertaken worldwide, no conclusive metallurgical understanding of this treatment has been established.
A wide range of experimental techniques has been applied to investigate the atomic displacement of carbon and microstructural changes in martensite during tempering, including X-ray diffraction, electron microscopy and diffraction, Mossbauer spectroscopy, atom probe field ion microscopy, electrical resistivity, dilatometric and calorimetric analysis. According to the present state of knowledge the structural evolution of martensite on tempering can be divided into the following sequence of processed: (a) the 0-th stage, the formation of carbon atomic clusters, modulated structures and orders structures, (b) the first stage, where the martensite decomposes into low carbon martensite containing 0.2 to 0.3wt% C and e-transition carbide particle, (c) the second stage, the decomposition of retained austenite into ferrite and cementite, (d) the third stage, conversion of the transition carbide into cementite and complete loss of the tetragonality of martensite.
Although a lot of works about tempering behaviour have been done, a complete and satisfactory understanding of the mechanisms of the structural changes involved has not yet been obtained. The 0-th stage, i.e., prior to carbide precipitation, and the first stage were of interest in the last decades. However, not so much attention has been paid to studying the effects of cryogenic treatments on the carbide precipitation in martensite during tempering.
The aims of the present study, therefore, are to investigate metallurgically the wear resistance and the microstructure of tempered alloy tool steels after quenching, after cold treatment at 223K and after cryogenic treatment at 93K.
2. Experimental procedure
Alloy tool steel with composition (wt%), 1.44C,0.3Si, 0.4Mn, 12.2Cr, 0.84Mo, 0.43V, 0.022P, and 0.008S was used. Heat treatment was performed at a constant heating rate of 0.17K/s up to 1073K in a vacuum furnace at 4x10 –3 Pa, then up to austenitizing temperature of 1293K or 1373K with a nitrogen atmosphere at 20 Pa, followed by quenching to room temperature and aging at 333K to avoid crack. The martensite start temperature (Ms) of this alloy is approximately 373K. Cold treatment at 223K and cryogenic treatment at 93K were carried out.
Figure 1 shows a typical heat treatment cycle of experiments. The specimens for the wear resistance with a size 25 x 50 x 10mm were ground and polished mechanically after tempering at 453K for 1.8Ks. A schematic diagram of a sample-on-wheel wear test machine is shown in fig. 2. A friction wheel, which has the same chemical composition as the specimens, was quenched and tempered with a hardness HV 780. No lubrication was used. The friction wheel rotated at a peripheral speed from .05 to 3.62m/s, the sliding distance was from 200 to 600m and the applied load was 21N. The wear rate, Ws=Bb-3 / 8rPL, was calculated, where B is the thickness of the wheel, b is width of wear, r is the radius of the wheel, P is the applied load, and L is the sliding distance. The volume fraction of retained austenite was determined by X-ray phase analysis at room temperature. Peaks (211) of martensite and peaks (311) of the retained austenite were employed. The thin sheets of approximately 100um in thickness for the observation of transmission electron microscopy (TEM) were austenitized and quenched by the same condition as mentioned above. The pressure of nitrogen atmosphere at 20Pa was controlled in order to avoid decarburization due to oxidation and / or evaporation during heat treatment. According to chemical analysis, the carbon content of the heat-treated sheets has been confirmed to be unchanged. Tempering was carried out at 453K for 600s after cold treatment and cryogenic treatment. The Theta 3mm discs were punched and jet-polished to perforation using an electrolyte of 10% HCIO4 and 90% CH3 COOH at 283K.
3.1 Wear Resistance
The Variation of the wear rate with sliding speeds is shown in Fig. 3 for specimens austenitized at 1293K, quenched and ultra-subzero treated at 93K. Finally tempering was carried out at 453K for 1.8Ks. The wear rate of specimens after cryogenic treatment is smaller than that of as-quenched specimens (without any subzero treatment) for whole sliding speeds. Furthermore, it decreases dramatically at high sliding speed. The cryogenic treatment results show 110 to 600% improvements. The wear rate shows a minimum at the sliding speed of 1.14 and 1.63m/s for specimens without and with cryogenic treatment, respectively. In Fig. 4, the variation of the wear rate with sliding distance is shown for specimens quenched, subzero treated at 223K and ultra-subzero treated at 93K. The wear rate as of quenched specimens is larger than that of specimens after cold treatment and cryogenic treatment at sliding distance of 200m. At sliding distance of 400m and 600m, the specimens after cold treatment have almost the same wear rate as quenched specimens. However, the specimens after cryogenic treatment have a smaller wear rate than as quenched specimens and specimens after cold treatment for any sliding distance.
3.2 X-Ray Diffraction Analysis
The volume fraction of retained austenite is plotted against the subzero treatment temperature in Fig. 5 for specimens austenitized at 1293K and 1373K. The volume fraction of retained austenite is 12% for as quenched specimens after austenization at 1293K, and approximately 6% for specimens after cold and cryogenic treatment. However, it decreases with treating temperature going down for specimens austenitized at 1373K. Cold treatments reduce the volume fraction of retained austenite drastically. Nevertheless, cryogenic treatment reduces it slightly relative to cold treatment.
3.3 Structure Observation of TEM
3.3.1. As quenched and tempered structure
The microstructure of as quenched specimens consists of mainly fine twinned martensite and retained austenite, the spacing between twins being a few tens nm. The bright field of Fig. 6 shows the microstructure of specimens quenched and tempered at 453K for 600s. The structure is mainly constructed with martensite and retained austenite. In the martensitic regions, many plates are internally twinned and some of the twins are extremely fine. The spacing between these twins being a few tens nm. On close examination, the twinned martensite shows coarse striated structure, which has been interpreted as the modulated structure produced by the spinodal decomposition, i.e., two variants of the carbon-rich regions. But there is a great difference from the results of Taylor and others. Only one set of fine parallel line contrasts was seen in the bright field image. The fine striations were spaced out about 1nm apart on average.
3.3.2 Microstructure after Cold Treatment
Fig. 7 (a) shows the bright field image of specimens subzero treated at 223K and then tempered. Course tweedlike structure corresponding to two orientation variants was present. This fine scale modulated structure has a wavelength of about 5nm. However, carbide cannot be observed in the modulated microstructure. Fig. (7) b and (7) c are the selected area deffraction pattern and the indexed pattern respectively, which consists of a (013) martensite zone. Inspection of deffraction pattern revealed streaking due to spinodal decomposition during the 0-th stage of tempering.
3.3.3. Microstructure after Cryogenic Treatment
The microstructure of the martensite after cryogenic treatment and tempering was remarkably changed, shown in Fig. 8 (a). In most of the areas appeared fine carbide particles developed in the boundary of twins. In different areas, fine carbide particles appeared at the points, which have a considerable diffusing density. Some rod like carbide particles parallel to each other appeared and varied in size from 5 to 10 nm in cross-section and from 20 to 40 nm in length. Fig. 8 (b) and 8 (c) are the selected area deffraction pattern and the indexed pattern, respectively. The deffraction pattern consists of a (111) martensite zone and a (101) n-carbide zone. It was seen that the (011) martensite plane is parallel to the (010) n-carbide plane, the (111) martensite direction is parallel to the (101) n-carbide direction.
From this pattern, it could be confirmed that the orientation relationship between martensite (x) and n-carbide is the Hirotsu and Nagakura relationship, as indicated below. (011) X //(010) n; (111) x //(101) n Jack identified transition carbide, which forms during tempering between 350K and 430K as hexagonal e-carbide. Table 1 compares the difference associated with the indexing of the carbide diffraction pattern according to published values of (d-hkl) for n-carbides and e-carbides. Although there is a close similarity between the structure of n and e-carbide, it is clear that the precipitated phase is n-carbide rather than e-carbide.
4.1. Relationship between Wear Resistance and Retained Austenite
From Fig. 5, the specimens austenitized at 1293K, subzero treated at 223K and ultra-subzero treated at 93K have almost the same volume fraction of retained austenite. However, the specimens after cryogenic treatment show wear resistance improvement considerably, as shown in Fig. 4. Although the specimens after cold treatment have a smaller volume fraction of the retained austenite than that of the as quenched ones, both have almost the same wear rate at sliding distances 400 and 600m. It is accepted that a major factor contributing to wear resistance improvement through subzero or ultra-subzero treatment is the removal of retained austenite and the formation of homogeneous martensitic structure although the hardness is hardly changed. According to the scanning electron microscopy observation of the worm surface of hardened carbon tool steel tempered at temperatures lower than 573K by Huet al., the predominant wear mechanisms were ploughing fatigue, fracture and delamination. In this case, the wear rate may be controlled by crack nucleation and propagation beneath the surface, which is related to the strength and toughness of the materials. Retained austenite may prevent crack propagation either by changing the growth direction of an advancing crack or by great energy absorption. It is suggested that cryogenic treatment makes a contribution to wear resistance due to fine n-carbides precipitation rather than the removal of retained austenite.
4.2. Mechanism of n-carbide Precipitation
A model of the bct-orthohombic system transformation is proposed. It is known that the lattice deformation of martensite results from cryogenic treatment. Fig. 9 (a) and (b) represent the relationship between (010) n-carbide plane and (110) martensite plane. This existence of the lattice correspondence between two phases implies that (010) n-carbide plane is derived from (110) martensite plane, and (100), (010) and (001) n-carbide directions are derived from (110), (110) and (001) martensite directions respectively. In the n- carbide structure, carbon atoms are in the octahedral interstices and iron or substitutional atoms take a hcp arrangement. The distance between neighbor iron atoms or subsitutional atoms in n-carbide and martensite is AB(n) >, Ab(x), BC(n) < BC(x). The lattice deformation is supposed to convert the parent bct lattice into an orthohombic n-carbide lattice through the readjustment of iron or subsitutional atoms due to contraction along (110) and (110) martensite direction of expansion along (001) martensite direction. Correspondingly, a slight shift of carbon atoms is required on (110) martensite planes in order to achieve carbon atoms stacking of n-carbide. This can be present as follows in bct lattice, carbon atoms at (1/2,1/2,0) and (1/2,1/2,1) positions in (110) martensite plane shift a/12(150), which may be explained a/6(110)+a/4(110), and ones at (0,1,1/2) and (1,0,1/2) position shift a/12(510), i.e. a/6(110)+a/4(110), where a means the lattice parameter of martensite. Furthermore, before contraction and expansion, a a/6(110) shuffling of iron or subsitutional atoms on alternate (110) martensite plane is necessary to meet the needs of stocking described n-carbide structure. Alternatively, iron or subsitutional atoms and carbon atoms in the other (110) martensite planes also change correspondingly as indicated above. Fig. 9 (c) shows the bct-orthohombic system transformation. The plain circle indicate iron or subsitutional atoms, the double circles indicate iron or substitutional atoms which belong to two unit cells and the solid circle indicate carbon atoms. The shuffling direction of atoms is shown by an arrow. It is suggested by TEM observation and crystallographic analysis of carbide that carbide nucleates heterogeneously along the carbon-rich bands which develop during the spinodal decomposition of martensite. It is well know that precipitation of fine n-carbide enhances strength and toughness of martensite matrix, and further increases wear resistance.
(1) Cryogenic Treatment increases wear resistance dramatically, especially at high sliding speed. The specimens after cryogenic treatment show a minimum of wear rate.
(2) Unlike cold treatment, cryogenic treatment promotes preferential precipitation of fine n-carbides.
(3) The formation mechanism of n-carbides is supposed to be as follows: iron or substitutional atoms expand and contract, and carbon atoms shift slightly due to lattice deformation as a result of cryogenic treatment.
(4) The mechanism that cryogenic treatment contributes to wear resistance is through the precipitation of fine n-carbide, which enhances strength and toughness of martensite matrix, rather than the removal of the retained austensite.
The authors wish to thank technician Mr. H. Yamamori and graduate student Mt. T Nakamura of Muroran Institute of Technology for their helpful cooperation in the experiment. Part of the investigation was carried out with the financial support of Senju Metal Industry Co. LTD.