HEAT TREATMENT RELATED TO WELDING

Welding as a joining method for fabrication of pressure vessels replaced riveting in early 1930's. In welding, joining is achieved by applying heat with or without the addition of the filler metal to produce a localized union through fusion. Ideally, welding a particular alloy with filler metal that matches exactly the composition. of base metal is expected but closer matching of mechanicalproperties (tensile, impact, creep etc.), freedom from cracking and in some cases resistance to corrosion are of greater importance. Thus in many cases the composition of welding electrode used is somewhat of a different chemical composition than the base metal. In this section the importance of metallurgical aspects have been outlined with respect to fusion welding of carbon and low alloy steels.

Cold Cracking

In the process of fusion welding, the weld and surrounding solid metal exposed to variable temperature gradient consist broadly of fusion / partly fused and heat affected zones (HAZ). The liquid weld pool solidifies to give a typical cast dendritic structure. In the HAZ, which remains solid throughout the welding process, the temperature reaches above ferrite to austenite transformation. When the steel cools down either after each run or completion of welding, transformation of austenite to ferrite takes place. The transformation structure formed in the weld or HAZ depends on chemical composition and rate of cooling. The T - T - T curves of base and weld metal can be used to determine types of transformation structure. For carbon steel up to 0.3%C" the structure will consist of ferrite and pearlite mixture. However, in case of higher carbon and low and medium alloy steels, the transformation product will be either martensite, bainite or mixture of martensite, ferrite, bainite and pearlite, depending on composition and rate of cooling. In most of such cases the solidified weld and HAZ will consist of hard and brittle phases. These high strength, low ductility structures are prone to cold cracking due to combined effect of a) stresses produced by shrinkage of weld and HAZ and phase transformation and b) hydrogen picked up by the weld and base metal. Role of hydrogen- is most important in this type of cracking. The mechanism can be explained as follows.

1. Molten weld pool absorbs hydrogen from electrode and from the environment due to reaction of hot metal with moisture.Fe + H20 -> FeO + 2H

2. Solubility of hydrogen in molten and solid iron changes with temperature and crystalline structure as shown in Fig V-13.

3. Much of the hydrogen absorbed by weld pool escapes as it solidifies but part of it also. diffuses into the HAZ.

4. Austenite has higher solubility for hydrogen than ferrite. As the cooling is comparatively fast, the transformed products formed from austenite remain supersaturated with hydrogen.

5. If the transformed microstructure of weld and HAZ is crack-sensitive and temperature below ductile to brittle transition temperature (normally below 150°C), hydrogen induced cracking occurs due to diffusion of hydrogen atoms around sharp flaws in presence of stress. Cold cracking can occur within minutes, hours or days after welding. The delayed cracks in many instances form after the weld inspection (radiography or ultrasonic) and go undetected

leading to later failures.

Carbon Equivalent

As cold cracking is related to crack sensitive microstructure, propensity of. cracking is related to composition of steel, conventionally converted into carbon equivalent (CE). Various empirical equations have been developed for plain carbon and low alloy steels. Plain carbon steels contain, in addition to carbon, manganese and silicon which effect the phase transformation behavior. The CE has been expressed in such steels as

Mn Si%C + ----- + ----

6 4If CE is plotted against cracking susceptibility, it is observed that as far as good weldability isconcerned, the CE for wrought material should be maximum 0.40 (Fig V-14). The figure also shows that cast steel has lower susceptibility to cracking than wrought material.

For both plain carbon and low alloy steels the following two relationship have been used.

(i) For carbon content of 0.12% or more

%Mn %Cu + %Ni %Cr + %V + %MoCE = C + -------- + --------------- + ------------------------

6 15 5

(ii) For carbon content of 0.07 to 0.22%

Si Mn+Cu+Cr Ni Mo VCE = C + ---- --------------- ----- ----- ----- + 58

30 20 60 15 10

To minimize cold cracking one or more of the following steps are taken, Le. (a) reduction in

residual stress due to shrinkage and joint restraint. b) reducing the. rate of cooling of weld and HAZ to form less susceptible micro-structure c) use of low hydrogen electrode d) avoiding welding under high humidity environment e) effusing out the hydrogen picked up during welding f) improving toughness and mechanical properties of transformed weld and HAZ structures and g) reducing, the residual stress after welding

Preheating

To important steps in reducing cold cracking are preheating and post weld heat treatment (PWHT). Preheating is an extremely effective method

ordinarily used to reduce (1) the cooling rates of weld and HAZ to form' more favourable microstructure and (2) the magnitude of residual shrinkage stresses. Various codes give the basis for determining preheating temperature but a broad approach can be stated as follows.

CE less than 0.45 % - optional preheatingCE more than 0.45%

but less than 0.6% - 93 to 204°CCE move than 0.6% - 204 to 317°C.

It may be mentioned that codes specify preheating and inter-pass temperatures (in case of high carbon and low alloy steels) based on both composition and thickness. The latter is important for low alloy steels where transformation to martensite takes place more readily. Interpose temperature should not be below preheat temperature and should also not be very high specially to avoid restriction on heat input and grain coarsening in the HAZ, which adversely affect its toughness/ ductility. Maximum preheat and interpose temperature normally should not exceed the recommended minimum for a particular steel by more than 80°C.

Post Weld Heat Treatment

Post weld heat treatment, (PWHT) forms an important part of welding process by which varieties of metallurgical objectives can be achieved. Some of the important effects of PWHT are a) stress relieving b) tempering of hard structure to improve toughness and mechanical properties and c) effusion of hydrogen absorbed by weld and HAZ. The PWHT temperature generally varies from 480 to 700°C depending on steel composition and end requirements but in any case the temperature should be below the entectoid transformation temperature or tempering temperature for quenched and tempered steel. In latter case too high a temperature may adversely affect the desired mechanical properties due to softening effect. Normally a temperature of 32°C below tempering temperature used during production is preferred. Stress relieving can be achieved at lower temperatures but required structural changes can be obtained in most of the cases, only at higher temperatures, where important controlling parameter is maximum limit on hardness of weld and HAZ. While selecting PWHT temperature this aspect should be taken into account.

For high strength quenched and tempered low alloy steel, where plate thicknesses are high, intermediate PWHT, (IPWHT) is followed during welding, to avoid hydrogen included cracking, The conventional IPWHT for 2.25Cr-1Mo steel is carried out at 600°C. To reduce the time factor of this procedure, in early 1980's, a new low temperatures intermediate PWHT (LTIPWHT), also known as low temperature dehydrogeneration treatment (LTDHT) was developed (about 350°C for 2 to 4 hours) to meet the primary aim of IPWHTT in reducing hydrogen to a safe level to avoid cracking, Fig V-IS compares the two methods of PWHT LTIPWHT, unless properly conducted to reduce the hydrogen below the threshold limit, can result in cracking. Sometimes micro cracks formed, can grow and coalesce over a period of operation, into macro cracks and final failure. This approach is also applicable for repair welds. Table V-2 gives a summary of two incidences of failure of identical ammonia converters having wall thickness of 125 mm using inadequate LTIPWHT in conjunction with hygroscopic aglomorated welding fluxes which resulted in higher hydrogen pick up during welding. Eleven other converters of essentially same design, welded with either low hydrogen fused flux with an 600°C IPWHT or stress relieved in a furnace did not show presence of any cracks .even after 16 years of service. L TIPWHT or L TDHT should therefore be carefully implemented to ensure absence of any cracks during fabrication or weld repair.

As already mentioned, maximum PWHT temperature is dependent on the nature of alloy and final desired mechanical properties of the weld. and HAZ. It is to' be ensured that mechanical properties do not fall below the minimum specified. This care is all the more necessary when necessity arises for re-PWHT, either after rewelding to remove any defect in original weld or for repair welding as a part of maintenance activity. The number of heat treatment cycles to which a particular equipment can be subjected should be given due attention. For example Cr-Mo steels used for fabrication of hydro cracker reactor is usually ordered on the basis of allowing for two additional PWHT cycles for future weld repair in the field.

Strees Relief Cracking

Many of welded low alloy steels containing strong carbide formers, e.g., Cr, Mo, V, Cb, Ti and Ta are susceptible to cracking during PWHT or when put into service at elevated temperatures. These cracks, also termed as reheat cracking, usually run parallel to the weld (Fig V-16) in the HAZ and sometimes in the weld metal.

The structure of HAZ IS not homogeneous and consists of four distinct regions with the one nearest to the weld having a coarse grain structure. This overheated and rapidly cooled coarse grain area retains almost all alloying elements in solid solution and therefore has higher strength but poor creep ductility. During PWHT, the locked in residual stresses are released by a creep / strain relaxation process. As grains are stronger, the grain boundary cavitations is facilitated leading to inter-granular failure. This process does not take place at a constant but decreasing stress. It is now accepted that presence of strong carbide former and presence of impurities segregated at grain boundaries enhance susceptibility to reheat cracking. It may be noted that the same compositional factors also adversely affect creep ductility and temper embitterment

Based on the industrial experience of fabrication and in-service cracking of Cr-Mo steels, extensive studies have been taken up during the recent years to define the compositional and welding parameters for code accepted fabrication and repair practices. The studies are being initiated primarily by American Petroleum Institute (API), Material Properties Council (MPC) and Pressure Vessel Research Council (PVRC) and some guidelines are' being arrived at.