INTRODUCTION
- Heat treatment is an important operation in the final fabrication process of many engineering components. Only by heat treatment it is possible to impart high mechanical properties to steel parts and tools for sophisticated applications.
- Heat treatment is considered to be very important tool of the metallurgist by which he can alter the properties of steel easily. The same steel can have a very wide range of mechanical properties if subjected to different heat treatment. Today when science and technology are advancing very rapidly in pursuit of higher and higher properties in materials, heat treatment plays a very important role.
PRINCIPLES OF HEAT
TREATMENT
- Heat treatment may be defined as a sequence of heating and cooling designed to get the desired combination of properties in the steel. The changes in the properties of steel after heat treatment are due to the phase transformations and structural changes that occur during the heat treatment. The factors, which determine and control these structural changes, are called the principles of heat treatment. The important principles of heat treatment are as follows:
- Phase transformations during heating.
- Effect of cooling rate on structural changes during cooling.
- Effect of carbon content and alloying elements.
POST WELD HEAT
TREATMENT
- High level residual stresses can occur in weldment due to restraint by the parent metal during weld solidification. The stresses may be as high as the yield strength of material itself.
- When combined with normal load stresses these may exceed the design stresses. The removal of residual stresses takes place due to the fact that the thermal energy received by the metal allows for grain boundary sliding and removal of metallurgical defects like dislocations, vacancies and slip planes. A most important aspect of Post Weld Heat treatment is the prevention of Brittle Fracture. Post weld heat treatment softens the hardened zones and makes the machining easy. Removal of residual stresses becomes necessary where dimensional stability is required. The heat treatment consists of the stress-relief, annealing or solution annealing depending upon the requirements.
THERMAL STRESS RELIEF
- Residual stresses resulting from welding are reduced by a post weld thermal stress relief heat treatment. The residual stress remaining in a material after stress relief treatment will depend on rate of cooling.
- The percentage relief of internal stress is independent of steel type, composition or yield strength. The effect of varying time and temper are shown in the graph below:
- The temperature reached during the stress relief treatment has a far greater effect in relieving’s tresses than the length of time the specimen is held at that temp. The closer the temperature is to the critical or re-crystallization temperature, the more effective it is in the removal of residual stresses.
- When a thermal stress relief treatment is employed to reduce residual stresses, other important properties must be taken into account. The micro-structure, tensile and impact strength are among the properties affected by the Stress relief treatment.
- A post weld heat treatment at 1040° C – 1090°C, spheroidizes the ferrite present in type 347 SS weldments, which effectively reduces sigma formation. This treatment also takes some of the ferrite into solution. An intermediate hold at 595° C is recommended for those weldments to relieve residual stresses and to reduce the susecptibity to cracking during post weld heat treatment.
- The heat affected zone (HAZ) in the vicinity of welded joints are aided considerably by post weld heat treatments. The properties of those zones are improved by the reduction of residual stresses together with the metallurgical changes brought about by the H.T. If any dissolved hydrogen is present it is given an aided opportunity to escape.
- The necessity for post heating increases with higher carbon content, increased alloy content and cross-sectional thickness of the part.
- The temperatures recommended for stress relieving low carbon steels are 595° C to 675° C. One hour per inch of thickness is the basis used to determine the length of time at the desired temperature. Larger periods of time are required at 595° C to achieve the same degree of stress relief as of 675° C.
- Thermal stress relief can be conducted in any furnace suitable for heating of the entire weldment. Same time only a portion of the weldment is heated if the structure is uniform in cross-section and the unheated ends are free to move and do not restrict the expansion of the heated part.
- The heating and cooling must be gradual and at temperature change. Because of variable factors such as strength characteristics of the metal and geometry of the parts, it is not possible to recommend formula for max heating or cooling rates. The following rules can be used as an approx. guide for simple cases:
Case 1
- Where the ratio of max to min thickness does not exceed 4 to 1 heating or cooling rate should not exceed 200° C/hr divided by the max thickness in inches at the weld.
Case
2
- Where the ratio of maximum to minimum thickness exceeds 4 to 1, heating or cooling rates should not exceed
1
inch thick – 95° C/hr
2
inch thick – 65° C/hr
3
inch thick – 38° C/hr
4
inch thick – 27° C/hr
5
inch thick – 16° C/hr
6
inch thick – 10° C/hr
- For complicated structures with members of widely varying thickness the safest procedure is to attach thermo-couples to all critical sections and not permit more than 42°C difference between any two sections.
- In this paper six case studies of post weld heat treatment for nuclear components have been discussed.
CASE STUDIES
12” NB Carbon Steel Pipe:
- 12” NB Carbon Steel pipe have been welded by GTAW required for Kaiga Atomic Power Project. After joining/welding, thermal stress is developed, to reduce their thermal stresses post weld heat treatment done.
- The welded pipe, after cleaning the joint has been loaded in a localized set up furnace and pre-heated to 300° C and then temp raised to 625°C to 650° C at the rate of 150° C/hr (max)
- Soaking time: 45 minutes
- Rate of cooling 150° /hr cool up to 300° C and then still air cool. The treatment has been done to achieve full strength.
12”
NB C.S. Elbow:
- 12” NB C.S. Elbows welded by GTAW required for Kaiga Atomic Power Project. After welding post weld heat treatment have been done.
- The job loaded in BHF and pre-heated to 300° C Temp raised to 625° C to 650° C at the rate of 150°/hr
- Soaking time: 45 minutes
- Rate of cooling 150° C/hr cooled up to 300° C then still air cool.
- The purpose of the treatment was to achieve full strength.
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| 12” NB C.S. Elbow |
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| 12” NB C.S. Elbow |
DETECTOR FRAMES FOR STATION
II - PHENIX
- CDM was assigned to develop and manufacture North and South hanging structures (for N & Sarm magnets) out of Aluminium Alloy 6061 T6. Each of these structures consists of 4 Front and 4 Rear chambers and each of these chambers is made up of a pair of Al. Alloy Frames. To develop the manufacturing procedure, one North side Front chamber was built on prototype basis, adopting weldment route for Aluminium structures, welded structure calls for post-weld heat treatment (Stress relief) for which a special furnace, of triangular shape with 2.5 m long slides was built. The prototype chamber was inspected and approved by Los Alamos National Labs, USA at CDM.
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Phenix Aluminium Frame |
- Solution annealing of the frame carried out (Fr. Stress relieving to effect solid solution of alloying constituents (e.g. Mg, Cr, and Si) and to improve mechanical properties of Aluminium alloy.
- Material: Al 6061 (Al, 1.0 Mg, 0.25% Cu, 0.6% Si, 0.20% Cr)
- Soaked for 2 hrs at 55° C ± 5° C and then water quenched.
- Aging (precipitation hardening) (to be done within 18 hrs of solution treatment) at temp. 170° C ± 5° C
- Soaked for 12 hrs and still air-cooled. (Hardness obtained 100 – 120 BHN).
- During this process precipitation of soluble constituents (e.g. Mg, Cr, Si) from Super saturated solid solution takes place. As this precipitation progress the strength of material increases.
STRESS RELEIVING OF SS316
AFTER STELLITE DEPOSITION
- Pipe loaded in furnace at 300°C pre heat temp of and heated to 900°C (rate 150°C/hr) soaked for 30 minutes at 900°C and allowed to cool at rate of 150°C/hr up to 400°C and then furnace cool. This operation was carried out to obtain full hardness and uniform stresses.
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PFBR Sleeve |
POST WELD HEAT
TREATMENT OF ALUMINIUM SERVO – DRIVE HOUSING
- Material: Al LM-14
- Composition: Al, Cu: 3.5% to 4.5% Si: 0.7 %, Mg – 1.2 to 1.8% Cr < 0.21%
- Solution Heat treatment – Temp. 515° C ± 5° C for 6 hrs and then Air blast quenching
- Aging (Precipitation Hardening) Temp: 232° C ± 5° C for 1 to 3 hrs and cool in still air.
- T.S: 215 to 280 Mpa
- Hardness: 100 BHN (approx.)
- This has been done for stress relieving of the housing.
POST WELD HEAT
TREATMENT OF SS 430 (Martensitic) + SS 304 (Austenitic) WELD
- Job heated at the rate of 100° C/hr up to 700° C 20° C and soaked at this temp for 5 hrs. Furnace cool at rate of 30° C/hr up to 595° C and then cool in still air. This operation carried out to achieve impact properties.
SEAL DISC
- Seal disc is used to prevent escape of heavy water from the coolant channel. The seal disc consists of a disc made of 17-4 PH heat treated to H 1100 condition to which is bonded an annular disc of nickel, about 1.5 mm thick by electro deposition from an aqueous bath. Sealing is accomplished by deflecting the sealing disc on the face of the end fitting of the coolant channel and then locking the deflection. Nickel serves as a sealing gasket. For efficient sealing the nickel should be soft (hardness not more than 170 HV) and the bond between nickel and SS should be adequately strong.
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Seal Disc |
- The seal disc is heat treated after Nickel diffusion bonding as per procedure stipulated below. Seal disc are given solution treatment and precipitation hardening. Seal disc are heated in furnace to 1050°C at the rate of 25-50° C/hr and held for 30 minutes. Thermo-couples are fitted at the ends and the middle zone. The job then quenched in oil at room temp. 30° C. Subsequently quenched in cold water at 5-10° C. Then precipitation hardening at H1075 (570 to 585° C) carried out.
- This has been done to achieve appropriate conditions prior to diffusion bonding.
CONCLUSION
- Post weld heat treatment is necessary to satisfy one or more end requirements. Every case has to be independently treated.
Authors
Mr Khaleel Ahmed
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