SELECTION OF WELDING CONSUMABLES FOR POWER PLANTS
Our country's economic reforms are encouraging' investments in power generation, refineries, petrochemicals, telecommunications and information technology. Among these sectors the power sector as an industry provides very good scope for welding and allied activities, Over the years many sophisticated varieties of steels have come to play its part. Simultaneously welding methods have evolved to make the joining and fabrication of metals, because of which welding plays a very prominent role in the power plants.
Apart from structural and other fabrication, welding was also used for maintenance applications of various power generation and other allied equipment, In maintenance it is mostly used for hardfacing, reclamation and general repairs. Now reclamation has become an economic necessity to conserve expensive materials and to reduce inventories with the advent of modern welding technology, innovations in fabrication techniques and tailor made welding consumables suiting any welding requirements.
The SMAW process still contributes in large proportions in power plants even though general semi automatic and automatic processes are available today, since it has got certain typical advantages, which are well known to everyone.
This paper highlights the selection of electrodes for welding various materials. It also discusses the emerging materials to suit specific job requirements. This paper also highlights reclamation and maintenance applications of various jobs done by us at various power plants and standardized the products.
2.0 MATERIALS USED IN POWER PLANTS
Various standards like ASTM, ASME, AISI are available for selecting materials table-1 for various requirements. In general, the materials used in the power plants can be grouped broadly under the following heads.
a. Carbon Steels
b. Carbon - Molybdenum Steels
c. Chromium - Molybdenum Steels
d. Stainless Steels.
The electrodes for various grades in each group and the welding intricacies of these materials are discussed in the subsequent paragraphs.
3.0 CONSUMABLES FOR FABRICATION
Carbon steel forms a major group of materials used in power plants. The welding of mild steel rarely poses any problem. Normally rutile (or) basic coated electrodes are used. For one side welding applications like E6010 for root run and E6013 for subsequent welding are used and when thickness of the material increases, the low hydrogen type electrodes like E7018, E7016, E7018-1 are also used to get good ductility and sub zero impact strength. Some time even continuous consumables like ER70S6 and ER70S2 were also used.
4.0 ELECTRODES FOR C - Mo AND Cr - Mo STEELS
This group of steels forms a critical group of materials used in power plants. The addition of Cr and Mo imparts good creep resistance, scaling resistance to the steel. Table-2 gives the maximum scaling temperatures for various materials, which are used in power plants. Most of the requirements like subzero impacts, control over hardness, more PWHT soaking time, X- factor and J-factor are met without any difficulty.
List of consumables based on materials composition is shown in the table-3. It was established that the creep resistance of Cr - Mo could be improved further by addition of carbide forming elements like Nb and V. The steel, which contains these elements, are called modified Cr - Mo steels. Cr - Mo family latest developments are discussed in the following paragraphs.
5.0 MODIFIED 9Cr - 1 Mo STEEL (P91)
The modified 9Cr-1Mo grade known as P91rr91 was introduced in the 1980s and recognized all over the world and used in high temperature applications when the temperature exceeds 5400 C and pressure is high. The steel contains 0.10%Nb and 0.25%V in addition to 1%Mo which enables it to achieve sufficiently higher creep strength than P22. The common material specifications are X10CrMoVNb91, SA 199 GrT91 , 213T91 and SA-335TP1/P91 etc. The chemical composition is shown in table-4.
Such composition associated with the proper heat treatment gives a fully martensitic structure leading to the required mechanical properties at room temperature are shown in table-5. 'Ni' decreases the Cr-eqv. Stabilize maretensitic structure and increase the toughness up to a certain limit but excessive 'Ni' decreases the toughness. For this reason impact properties have a maximum value at an addition of about 0.5%Ni. 'Nb' and 'V' increase the creep strength and decrease the toughness. 'N' increases the creep strength as well as tensile and yield strength.
The modified 9Cr-1 Mo steel has the higher allowable stress compared with conventional steels because of this section thickness and overall weight of the component can be reduced. Suitable consumables were developed and its chemical composition and mechanical properties were shown in table-4 & 5. Comparative details of P91 with other materials are shown in Fig-1.
6.0 9Cr - 0.5Mo - 1.8W - V - Nb (P92) STEEL
The investigation was carried out mainly in Japan by adding 'W' together with 'Mo' and found that higher stress rupture strength compared with modifiedP91!T91(NF616) Fig.-2. The 'W' containing P91 material was called as P92 and used for Headers, Steam pipes etc. The detailed information about this steel was incorporated in the ASTM Standard A-335 in March 1996. The addition of 'W' results in higher stress rupture strength even at 600°C over other candidate steels Fig.-3. The 'W' helps in solid solution strengthening and also reduces the rate of coarsening of M23C6 carbides.
The detailed chemical composition is shown in table-6. The P92!T92 steel has high allowable stress compared with other steels and hence, the component size and wall thickness is reduced by 25%. These materials are used in hardened and tempered conditions. The restriction of 'W' results improved fabricability, especially weldability. The optimum mechanical properties like strength and toughness can be achieved at a temperature of about approx.770oC.
The different fabrication codes, Specifications specify the mechanical properties and PWHT are shown in table-7. Suitable consumables meeting these specifications were developed and its properties are shown in table-6 & 7.
7.0 STAINLESS STEEL WELD METALS
Mostly austenitic stainless steels are used in power plants because of their resistance to corrosion, oxidation and scaling at elevated temperatures. Some of the stainless steels used and its consumables are shown in table-3. Apart from this other materials like 308H, 347H, 316H are also used. Here 'H' denotes higher carbon version in the range 0.04 to 0.08%
8.0 WELDING PROCEDURE REQUIREMENTS
When we are talking about Cr - Mo steels it is necessary to understand welding procedures to get desired properties.
8.1 PREHEAT AND INTER PASS TEMPERATURE
Preheat is basically used for reducing cooling rate by reducing the temperature gradient between base metal and weld metal. Preheat is essential for all C - Mo and Cr - Mo low alloy steels to increase toughness, ductility of the base metal & weld metal and reducing the cracking tendency. Table-8 gives the details of minimum preheat required for various steels specified commonly used codes.
8.2 POST WELD HEAT TREATMENT
Post weld heat treatment is carried out to relieve the residual stresses in the weld as well as the heat affected zone, so that it can improve toughness and ductility. Table-9 details the recommended PWHT temperature for various steels.
9.0 MAINTENANCE APPLICATIONS
A list of various weld metals used in power plants are highlighted in the application wise in the following table.
The power plants use a number of materials varying from simple carbon steel to critical alloy steels. They call for the use of different electrodes meeting the desired properties. In general electrodes meeting the standard properties are suitable for various applications. However, the weld metals may also require modifications in order to meet specific job, fabrication requirements.
Successful welding of these materials calls for due attention to procedural aspects like preheat, IPT, PWHT, etc. A judicious choice of which will help in achieving the desired properties consistently. There are so many jobs, which are to be reclaimed at power plants. These jobs are right from the simple ferrous materials hard facing to the critical jobs. Joint efforts of maintenance, fabrication in power plants with the consumable manufacturers have resulted in development of several consumables for joining, repair and reclamation of numerous power plant machineries and components, establishing welding techniques method for various jobs.