METALLURGICAL CHANGES IN STEELS DUE TO CRYOGENIC PROCESSING & ITS APPLICATIONS

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METALLURGICAL CHANGES IN STEELS DUE TO                  CRYOGENIC PROCESSING & ITS APPLICATIONS

            Metallurgical changes in steels due to cryogenic

                          processing & its applications

                 

Abstract:  

Cryogenic processing is a supplementary process to conventional heat treatment process in steels. It is an inexpensive one time permanent treatment affecting the entire section of the component unlike coatings. Though the benefits have been reported widely, there are issues debated upon, in respect of the treatment parameters, extent of benefits experienced in different materials, underlying mechanism and pretreatment conditions. A study on the improvement in wear resistance and the significance of treatment parameters in different materials has been made. It is found that cryogenic treatment imparts nearly 110% improvement in tool life. It is even superior to tin coatings. The underlying mechanism is essentially an isothermal process.

Keywords: Cryo processing; Wear resistance

        

                 

INTRODUCTION: 

The word Cryogenics is derived from the Greek words 'Kryos" (meaning cold) and "Genes" (meaning born). The cryogenic processing is modification of a material or component using cryogenic temperatures.  Cryogenic temperatures are defined by the Cryogenic Society of America as being temperatures below 120K (-244F, -153C).  

Cryogenic processing makes changes to the crystal structure of materials.  The major results of these changes are to enhance the abrasion resistance and fatigue resistance of the materials.

           

 The thermal treatment of metals must certainly be regarded as one of the most important developments of the industrial age. One of the modern processes being used to treat metals (as well as other materials) is cryogenic tempering. Until recently, cryogenic tempering was viewed as having little value, due to the often brittle nature of the finished product. It is only since the development of computer modeled cooling and reheats curves that the true benefits of cryogenically treated materials have become available to industry and the general public. Cryo tempering is a permanent, non-destructive, non-damaging process (not a coating) which reduces abrasive wear (edge dulling), relieves internal stress, minimizes the susceptibility to micro cracking due to shock forces, lengthens part life, and increases performance. Cryo treated pieces are also less susceptible to corrosion. The deep cryogenic tempering process is a one-time, permanent treatment affecting the entire part, not just the surface.

In Ferrous metals, cryogenic processing converts retained austenite to martensite and promotes the precipitation of very fine carbides. Fine carbon carbides and resultant tight lattice structures are precipitated from cryogenic treatment. These particles are responsible for the exceptional wear characteristics imparted by the process, due to a denser molecular structure; reducing friction, heat, and wear. Cryogenic Processing is not a coating.  It affects the entire volume of the material.  It works synergistically with coatings. Furthermore, the cost of cryogenic treatment is said to be less than the cost of coating, which is currently a popular method for improving tool life. Cryogenic Processing has a great effect on High Speed Steel cutting tools.  The normal result is that the tools will last considerably longer, typically 2 to 3 times longer. Cryogenic processing establishes a very stable piece of metal that remains distortion free.  The process will also stabilize some plastics. The stamping, forming and cutting die industry is one of the first places where cryogenic processing worked its wonders. Cryogenically treated metals form better. Valve spring life can be improved up to seven times over the shot peened life by the use of cryogenics.

         

Cryogenic processing tinkers with materials at the molecular level at cryogenic stillness, resulting in:

  • Homogenizes the Crystal Structure

  • Grain Structure refinement

  • Improved structural compactness

  • Prevents concentrated Heat Built-up

  • Increases Resistance to Deformation

  • Reduces Deformation significantly

  • Retained austenite is converted to a fine martensite matrix

  • Mechanical Properties like micro-hardness, Tensile Strength etc. are the same across any cross-section

  • Significant improvement in dimensional stability

  • Relieves residual Stresses

  • Several fold improvement in hot hardness

  • Significant improvement in material toughness

  • Binder Materials like Cobalt Nickel and in some cases additives of tantalum,

  • Tungsten or Titanium are advantageously affected

  • Big decrease in the amount of catastrophic shattering

  • Produces stronger, denser parts for better performance and longer service life

There is no official definition of the process, the process parameters vary widely from one company to the next. With the use of cryogenically treated M7 high-speed steel drill bits for drilling holes in titanium alloys, the estimated annual savings was $350,000 for $1,000,000.

Increase productive life of engineering components by 25-100%

• Decrease perishable tooling consumption by 25% and add to profits

• Increase service life of tools by 50-200%

2. Cryogenics at a glance:

Cryogenics is the study of how to get to low temperatures and of how materials behave when they get there. Besides the familiar temperature scales of Fahrenheit and Celsius (Centigrade), cryogenicists use other temperature scales, the  temperature scales. One interesting feature of materials at low temperatures is that the air condenses into a liquid. The two main gases in air are oxygen and nitrogen. Liquid oxygen, "lox" for short, is used in rocket propulsion. Liquid nitrogen is used as a coolant. Helium, which is much rarer than oxygen or nitrogen, is also used as a coolant.

 In 1942, researchers at the Massachusetts Institute of Technology found that a certain favorable combination of properties could be achieved only by including a cold treatment in the processing cycle of a tool steel. Several years later, moderate to large improvements in tool steel performance were reported when cold treatments were used. A study conducted at Louisiana Technical University, indicated that holding at –310’F (-190’C) for longer times (20 hours, compared with 8, 10, 12, and 16 hours) produced greater improvement in wear resistance. That result probably accounts for the use of holding times of 1 or 2 days at the cryogenic temperature.

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     It has been observed that the process provides the materials a stronger, denser and more-coherent structure thus increasing the abrasive resistance and thermal and electrical conductivity. For steels, the explanation of the phenomena in Layman’s terms is as follows: Super cooling the steel refines the carbides in the steel by expanding the carbide structure to fill any voids in the metal. Then as the higher temperatures return, everything relaxes into where it wants to be thus providing stability to the steel. Every step in the treatment is carefully controlled else the temperature extremes will shock the steel into ...

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