Deep Cryogenic treatment of M2 Tool Steel

• The changes in the tensile strength and wear resistance resulting from cryogenic treatment
• The mechanism behind any property changes
• Model the phenomenon

From time to time over the last few decades, interest has been shown in the effect of low temperatures during the heat treatment cycle on the performance of steels, particularly tool steels. The major reason is improvement in the wear resistance property, which is very critical for the tool steels. A lot of claims for improvement in the property were published. These claims include:

1) Improvement in the wear resistance of carbide tools
2) Stabilization of the dimensions of aluminum parts
3) Extension of life in copper resistance – welding electrodes

But the understanding behind these property changes is still not very clear. Various mechanisms have been proposed but those are not yet accepted universally. These claims include precipitation of eta-carbides, conversion of retained austenite and stabilization of martensite.

There are mainly two kinds of low temperature treatments. A cold treatment, which is carried out at about –176°F, and deep cryogenic treatment that is carried out at –385°C (liquid nitrogen)

Cryogenic treatment we can say is an extension of the quenching process. Increase in the carbon content and alloy content leads to decrease in Ms Temperature and so the Mf temperature, which is generally below the room temperature for tool steels. By performing the low temperature heat treatment the transformation is completed and better stability in the properties are developed.

It is claimed that the advantage of using deep cryogenics is due to an enhancement of the precipitation of fine eta-carbides during the subsequent temper. The strain energy in the martensite lattice increases at a lower temperature. As a consequence Carbon atoms migrate and form clusters. During the subsequent heating back to the room temperature or even a tempering, these clusters act as nuclei for the formation of the ultra fine eta-carbides. Thus the steel product that results boasts improved hardness, toughness, wear resistance and resistance to fatigue cracking

The effectiveness of the cryogenic treatment at liquid nitrogen temperature was found to depend on
• The number of tempering cycles performed prior to freezing
• The use of either a high or low tempering temperature prior to freezing
• Use of tempering cycles after freezing as an extended quench

It was also claimed that as a result of cryo treatment there is an enhancement in the surface compressive stress

Source:http://tptc.iit.edu/members/Reports/2001-12/TPTC%20REPORT%20DEC%202001_Rajendra.pdf

Cryogenics goes Deeper

Although the application of deep cryogenics to cutting tools has yielded widely variable results in the past, current data from laboratory and field tests indicate that treatment can markedly increase tool life.

In spite of recent studies showing that deep cryogenics can improve the performance of cutting tools, the use of freezing processes to treat alloy-steel tools is debated in the metalworking community. In the 1950s and 1960s, when cryogenic treatment involved the direct immersion of tools in a medium maintained at -320° F, thermal shock typically damaged the tools. Memories of the unpredictable results of this earlier process have discouraged further R&D and use of cryogenic treatment.

As currently practiced, deep cryogenics does not directly expose cutting tools to cryogenic fluids. To minimize thermal shock, the tools are placed in an airtight refrigeration chamber (cold box) and the temperature of the tools is reduced gradually. The tools are then soaked in a dry cryogenic atmosphere at -310° F to -320° F for 20 to 60 hours, after which they are slowly returned to room temperature and subsequently tempered. The exact temperatures and times vary from manufacturer to manufacturer. Because this process is so time-consuming, it must be applied after heat-treatment. Without knowledge of the tools' heat-treat characteristics, the results of cryogenic treatment are completely unpredictable.

Comaprison of drilling thrust force for cryogenically and untreated twist drills when machining 4340 steel.

Drilling conditions and improvement results for laboratory test comparing cryogenically treated and untreated tools.

Source: http://www.ctemag.com/dynamic.articles.php?id=60

The Effects of Cryogenic Treatment on the Thermal Conductivity of GRCop-84

The mechanical properties of many materials have been enhanced via cryogenic treatment, which is a cold temperature process performed after traditional heat treatment. In this research the effect of cryogenic treatment on GRCop-84 was examined. Cryogenically and non-cryogenically treated samples were tested identically to determine whether cryogenic treatment has a significant affect on the thermal conductivity of GRCop-84. Optical and electron microscopy were used to characterize the material properties. Cryogenic treatment includes a temper that appears to be responsible for the enhancements observed in GRCop-84.

Cryogenic treatment is a cold temperature process performed after traditional heat treatment, not a substitute for proper heat treatment. Cryogenic treatment has been shown to enhance mechanical properties in many materials. The effect of cryogenic treatment on the thermal conductivity of a copper alloy developed by NASA (GRCop-84) was tested. Potential applications of GRCop-84 include regeneratively cooled rocket engine liners.
Source:http://web.ebscohost.com/ehost/pdf?vid=1&hid=104&sid=8f79dbbe-f760-4574-90f7-3e443e7cbffc%40sessionmgr104

Cryogenic treatment of production components in High-wear rate wells

"Deep Cryogenic Tempering (DCT) is a specialized processwhereby the molecular structure of a material is “re-trained” through cooling to –300º F and then heating to +175-1100º F. Three Shannon Formation wells were selected (TD about 500 ft) based on their proclivity for high component wear rates.

Phase 1 of the test involved operation for a nominal 120 calendar day period with standard, non-treated components. In Phase 2, treated components were installed andoperated for another nominal 120 calendar day period. Different cryogenic treatmentprofiles were used for components in each well. Rod pumps (two treated and oneuntreated) were not changed between test phases. One well was operated in pumped-offcondition, resulting in abnormal wear and disqualification from the test. Testing showsthat cryogenic treatment reduced wear of rods, couplers, and pump barrels. Testing ofproduction tubing produced mixed results."

Source: http://www.osti.gov/bridge/servlets/purl/794381-1aya1J/native/794381.pdf

Deep-Cryogenic Treatment of Steel

The deep cryogenic tempering process for gears is an inexpensive, one time, permanent treatment, affecting the entire part, not just the surface. Gears may be new or used, sharp or dull, and re-shaping will not destroy the treatment.

The process of deep cryogenic or freeze tempering is really quite simple. “A freeze tempering process means that the material to be treated is not exposed to any liquid nitrogen, which eliminates the risk of thermal shock. The material is frozen through a thermo dynamic refrigeration cycle. The material is cooled slowly, held for a prolonged period of time 48-60 hours, and allowed to return to room temperature slowly.”

Source: http://coldfire.com.au/uploads/Deep_Cryo_Article-Ferland.pdf

Frozen Gears

Durability is the most important criterion used to define the quality of a gear. The freezing of metals has been acknowledged for almost thirty years as an effective method for increasing durability or wear life, and decreasing residual stress in tools steels. The recent field of deep cryogenic (below-300°F) has brought high super conductors, the superconducting super collider, cryo-biology, and magneto hydrodynamic drive systems. It has also brought many additional durability benefits to metals.

Source:http://www.cryoeng.com/images/Frozen%20Gears.pdf

Cutting Tools Engineering

Although its effects on metal composition are subtle, deep cryogenic tempering can yield dramaticimprovements in tool performance.

In the search for cutting tool engineering that can increase productivity, prolong cutting life, and decrease costs, gains of 15% to 20% are considered significant. One recently developed tool treatment is showing far greater promise, in some cases improving tool life by 200% to 400%.

The method, called deep cryogenic tempering, subjects tools placed in a specially constructed tank to temperatures below -300 ° F for a number of hours using liquid nitrogen as the refrigerant. The process supplements standard heat/quench tempering, completing metallurgical changes that heat treatingbegins.

Source: http://www.cryosciencetechnologies.com/documents/cutting_tool_engineering.pdf