Department OF Mechanical Engineering

Einstein Academy Of Technology & Management,

Bhubaneswar

A SEMINAR REPORT ON

“Crogenics Heat Treatment of Metals”

(Under the supervision ofMr. Binayak Dash, Dept of MECHANICAL ENGG.)

Submitted by:

DEEPAK KUMAR SINGH

DEPARTMENT OF MECHANICAL ENGG.EINSTEIN ACADEMY OF TECHNOLOGY AND MANAGEMENT,KHURDA-752060

Content

ABSTRACT CRYOGEICS HISTORY PROCESS OF CRYOGENIC TREATMENT APPLICATION OF CRYOGENIC TREATMENT OTHER APPLICATION OF CRYOGENIC

TREATMENT

Abstract A cryogenic treatment is the process of treating work pieces to cryogenic temperatures (i.e. below −190 °C (−310 °F)) in order to remove residual stresses and improve wear resistance on steels. In addition to seeking enhanced stress

relief and stabilization, or wear resistance, cryogenic treatment is also sought for its ability to improve corrosion resistance by precipitating micro-fine eta carbides, which can be measured before and after in a part using a quantimet.

The process has a wide range of applications from industrial tooling to the improvement of musical signal transmission. Some of the benefits of cryogenic treatment include longer part life, less failure due to cracking, improved thermal properties, better electrical properties including less electrical resistance, reduced coefficient of friction, less creep and walk, improved flatness, and easier machining

The deep cryogenic treatment and tempering process for metals is economical. It is a one time permanent treatment, affecting the entire part, not just the surface. The treatment may be applied to new or used tools, sharp or dull, and reshaping will not destroy the imparted properties. Benefits achieved from subjecting tools to this treatment include: increases in tensile strength, toughness, and stability through the release of internal stresses.

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 (more information in the ‘About The Process’ section).

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’

Cryogenic Processing:-The cryogenic processing is modification of a material or component using cryogenic temperatures. The workers at the National Institute of Standards and Technology at Boulder, Colorado have chosen to consider the field of cryogenics as that involving temperatures below –180C(93.15 K) . Cryogenic processing makes changes to the crystal structure of materials.

Deep sub-zero (much below 0C) processing of metals and alloys is a deep stress relieving technology. The third law of thermodynamics states that entropy is zero at absolute zero temperature. Cryogenic processing uses this principle to relieve stresses in the material. The materials are subjected to extremely low temperatures for long period of time leading to development of equilibrium. This leads to decrease in defects in the material and it attains a minimum entropy state.

Cryogenic processing will not in itself harden metal like quenching and tempering. It is not a substitute for heat-treating. It is an addition to heat-treating. Most alloys will not show much of a change in hardness due to cryogenic processing. The material will have to be cryogenically treated followed by tempering to gain the hardness and toughness.

CRYOGENIC TREATMENT:-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. Cryogenic treatment is a one-time permanent treatment process and it affects the entire cross-section of the material. Usually done at the end of conventional heat treatment process but before tempering. Also it is not a substitute process but rather a supplement to conventional heat treatment process.

Classification of Cryogenic Treatment

Cryogenic treatment has been classified into shallow cryogenic treatment (SCT) and deep cryogenic treatment (DCT) depending upon the temperatures in which the material is treated:

SCT- tool steel is keep in freezer at 193K for 5 h and then exposed to RT

DCT- material is brought down to 77K at 1.26 K/min, held there for 24 h and brought back to RT at 0.63 K/min.

Theories for Cryogenic Treatment

The researchers have devised following possible theories responsible for the changes in properties of tool steel after cryogenically treated:

Complete transformation of retained austenite into martensite. Precipitation of microscopic carbides into martensite.

Fig 1. Microstructures showing transformation of retained austenite to martensite [20]

The martensite formed is darkened by subsequent tempering, the light areas are martensite plus retained austenite. Steel M-182, etched with 1% nital + 1 % zephiran chloride. X750.

(a) transformed at 265 C, 62% martensite;

(b) transformed at 240 C, 81% martensite;

(c) transformed at 210 C, 93% martensite 8

In conventional heat treatment cooling is done till RT, which leaves some retained austenite in the microstructure. This retained austenite is soft and unstable at lower temperatures and thus transforms into martensite. The martensite formed has a 4% volume expansion which causes distortion.

Fig 2. Change of crystal structure on transformation of austenite to martensite

The Mf temperature of eutectoid steel is app −50 °C, and retained austenite is present after quenching. If the material is submitted to reheating or to a stress field, transformation to martensite will take place causing distortion on its body. This non-tempered martensite may cause cracks but the subzero treatment will transform a great deal of this retained austenite by reaching the Mf line, giving more dimensional stability in the tool microstructure. stability in the tool microstructure.

Fig 3. TTT diagram of eutectoid steel [21]

Time-temp curve showing the formation temp (Mf) of martensite from

metastable austenite and the path followed to form tempered martensite

The influence of precipitated particles is shown by yet another research done on M2 steel by varying the cryogenic cycles. Their research involved seven steel samples, each of them submitted to different heating and cooling (up to −70 °C) cycles. The microstructure was analyzed and the carbide particles quantified using SEM, X-ray difractometer, quantitative metallography and differential

dilatometer. The results confirmed an increase in carbide precipitation (from 6.9% to 17.4%), a reduction of the retained austenite (from 42.6% to 0.9%) and an increase in the martensite content (from 66% to 81.7%).

Barron compared the improvement in wear resistance after cryogenically treating M2 high speed steel at −84C (for 24 h) and at -196C and observed small change in amount of retained austenite, but large increment in the wear resistance. Here the untreated microstructure showed large carbides (20 μm) dispersed in the matrix which converted to small particles (5 μm) after treatment. This suggests the presence of hard and small carbide particles well distributed among the larger carbide particles within the martensite matrix increases the wear resistance.

3.2 Making of Liquid Nitrogen

Liquefied gases, such as liquid nitrogen and liquid helium, are used in many cryogenic applications. Liquid nitrogen is the most commonly used element in cryogenics and is legally purchasable around the world. Liquid helium is also commonly used and allows for the lowest attainable temperatures to be reached

Table 1. Boiling temp of different cryogens [

S No Element Boiling Temp

1 Oxygen –183 ◦C2 Nitrogen –196 ◦C3 Neon –247 ◦C4 Hydrogen –253 ◦C5 Helium –269 ◦C6 Carbon

dioxide–80 ◦C

A common method for production of liquid nitrogen is the liquefaction (phase change from gaseous to liquid) of air. In the liquid nitrogen compressors air is compressed, expanded and cooled via the Joule-Thompson’s effect.

Fig.4 Difference in the boiling temperature of oxygen and nitrogen distilled out of the liquid air

Cryogenic Treatment Procedure

The liquid nitrogen as generated from the nitrogen plant is stored in storage vessels. With help of transfer lines, it is directed to a closed vacuum evacuated chamber called cryogenic freezer through a nozzle. The supply of liquid nitrogen into the cryo-freezer is operated with the help of solenoid valves. Inside the chamber gradual cooling occurs at a rate of 2 C/min from the room temperature to a temperature of -196C. Once the required temperature is reached, specimens

are transferred to the nitrogen chamber or soaking chamber wherein they are stored for 24 hours with continuous supply of liquid nitrogen.

Fig 5. Liquid Nitrogen System (Gas Cooled)

A Typical Cryogenic Cycle:-

1. RAMP DOWN:- Bring down the temp to -184C over a period of 6-10 h to avoid thermally shocking the part. Reason for such huge time interval can be think in terms of dropping a cannon ball into a vat of liquid nitrogen. The outside of the cannon ball wants to become the same temp as the liquid nitrogen, which is near 76K (-196C) and inside wants to remain at RT. This sets up a temp gradient that is very steep. The cold area wants to contract and the inside wants to stay at RT. This sets up stresses on the surface, which leads to cracking at the surface.

Figure 7. Cryogenic Treatment Cycle Practiced By NFC, Tool Room [23] 11

2. SOAK: soak segment will hold the temperature at 123K (-150C) for 8-40 h. Crystal structure of the metal changes at this temp at a slower rate. One of the changes is the precipitation of fine carbides. In theory a perfect crystal lattice, structure is in the lowest energy state. The total energy in the structure is higher with vacancies and dislocations. By keeping the part at a low temperature for a long period of time, we get some energy out of the lattice and make a more perfect and therefore stronger crystal structure

3. RAMP UP: A typical ramp up segment brings the temperature back up to room temperature in 8-20 h. The ramp up cycle is very important to the process. Ramping up too fast can cause problems with the part being treated as happens by dropping an ice cube into a glass of warm water, it cracks.

4. TEMPER RAMP UP: temper segment ramps the temp above the ambient to a predetermined level over a period of time. Tempering is important with ferrous metals. The cryogenic temperature will convert almost all retained austenite into primary martensite, which is brittle. To reduce the brittleness it is tempered back using the same tempering process as is used in a quench and temper cycle in heat treatment. We ramp up the temp slowly to assure the temp gradients within the part are kept low. Typically, tempering temp are from 422K (149C) on up to 866K (593C), depending on the metal and required hardness

5. TEMPER HOLD: Holding the elevated temperature for a specific time. The temper hold segment assures the entire part has had the benefit of the tempering temperatures. A typical temper hold time is about 3 hours. This time depends on the thickness and mass of the part. There may be more than one temper sequence for a given part or metal. We have found that certain metals perform better if tempered several times.

Effects of Cryogenic Treatment:-

This treatment results in significant amount of retained austenite which has some damaging effects on mechanical properties of tool steels such as machinability, wear, hardness and most important of all on dimensional stability of tool steels. The latter could be a very significant factor in the case of using the tool steels for die material applications. Thus different treatment cycles were applied on the samples to study the effects of low temperature treatments on tool steel. This was done by cooling the samples at temperatures well below the M f temperature of the tool steel and holding the samples in this temperature range.

Fig 1 - The annealed tool steel samples used in this investigation were held at 1040 °C for 30 min for austenitizing followed by air quenching before any treatment. This is the conventional hardening treatment of M2 tool steels and the microstructure of the alloy based on the phase diagram consists of needle type martensite, retained austenite and carbides. [27]

Fig 2 - scanning electron microscopy images of the microstructure of alloys 1–4 after different heat treatment sequences. The execution of cryogenic treatment had a significant effect on the microstructure of the alloy and led to transformation of retained austenite to martensite.

Fig 3 - As the cryogenic temperature is lowered, more austenite is transformed to martensite. X-ray pattern of the alloy after conventional treatment is shown in the figure.

For the calculations, peaks (2 0 0) and (2 1 1) of martensite and (3 1 1) and (2 2 0) of austenite were employed. The lower angle peaks (1 1 0) of martensite and (1 1 1) of austenite were neglected because they appear very close to each other which could introduce errors in the evaluation of their integrated intensities[26]. The measurement of percentage retained austenite in the microstructure (Table 1) also verifies the transformation of retained austenite to martensite. Furthermore, the transformation of austenite to martensite results in the volume increase which causes tensile stress on austenite.

Retained austenite volume content after different treatment cycles.

Sample Retained austenite percent (%)

1 8.1

2 7.4

3 4.5

4 3.8

ADVANTAGES OF CRYOGENIC TREATMENT OF METALS:-

Researcher has shown many benefits, following are the main benefits of cryogenically treated material: Significantly enhances abrasive wear resistance Improves corrosion resistance Increases dimensional stability before extensive machining This process is irreversible, with beneficial properties retained even after

resurfacing Closes and refines metal grain structures Transforms retained austenite into the harder, more desirable martensite Reduces retained stresses, as well as wear and surface stresses Cuts operating costs and downtime by reducing the need for tool

replacement or regrinding and maintenance

Effective for items ranging in size from tiny drills to dies weighing hundreds of pounds.

DISADVANTAGES High cost of operation Processing Difficulties( Temp. Difficult to Maintain) Area Under Research Difficulties To manufacture The Equipments Standards not followed in INDIAN Scenario

APPLICATIONS

Cryogenic processing has a huge range of applications in materials processing.

Just a few of them are: Shrink fitting is a convenient technique to assemble parts when the

tolerances between pieces are very close or there is a negative tolerances. The shaft is cooled to decrease its diameter; the shaft is then assembled with its related parts and allowed to warm up to room temperature to make the join.

Stabilization of aluminum and magnesium alloys can be achieved by repeated cycles of cryogenic soaking and tempering. Cycling occurs from 3 to 10 times between –185 °C and +125 °C. The rapid cycling of temperature reduces residual stresses caused by dislocations and internal defects and promotes dimensional stability and machinability.

Cryogenic wire drawing When austenitic stainless steel wire is drawn at cryogenic temperatures instead of room temperature there is improvements of 30 % in yield and ultimate tensile strength.

The major use of cryogenic treatment in the processing of materials is to improve the properties of steels.

REFERENCE

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