Welding and Post Weld Heat Treatment of WB 36 Steel

S. Singaravelu, Former AGM BHEL, Tiruchirappalli, India
The need for increasing the plant efficiencies for fossil fuel fired power plants necessitates development and use of new materials in boilers. Emission rules demands reduction of CO2 emissions and thus leads to higher operating pressures and temperatures. Once through Super Critical Boilers (OTSC) manufacturing involves higher quantum of pipes to transport high pressure and high temperature water and steam. In order to reduce the piping weight and the related supports and hangers, new material WB 36 /P36 is used in boiler feed water piping. This steel grade is mainly used in high-pressure, medium-temperature feedwater lines (close to 400 °C).WB36 exhibits excellent high-strength properties up to 450 °C. The reduced weight of the feed water piping components compared with conventional carbon steel allows improved resistance to thermal fatigue and high level of mechanical characteristics at the service temperature. The aim of this study is to demonstrate joining of thick wall piping and to qualify a Welding Procedure Specification (WPS) by GTAW with SMAW process. Post Weld Heat Treatment (PWHT) was carried out to assess the required weld metal strength, hardness and toughness.
Key Words: OTSC, WPS, PWHT, Toughness, Hardness.
1.0     Introduction
Increasing of operating parameters (pressure, temperature) and unit size of thermal boilers requires the development of high strength steels. Developments in steelmaking have improved the cleanliness and properties of steel. In Sub critical boilers high pressure hot water are present primarily in feed water system and headers, which are manufactured by standard carbon steels like A106 grade B or A106 grade C pipes. For super critical power plants, this leads to very thick piping system. V&M has developed WB 36 steel (15 NiCuMoNb5-6-4) for high pressure piping of boiler feed water system. It is designed to compete against the carbon manganese pipes. It is a low alloy steel with 1.1 % Ni and 0.7 % Cu. Addition of copper increases the strength level. WB 36 seamless pipe became significant when the capacity of the newly built power stations is increased. In the feed water applications based on ASME B 31.1, the main advantage is the possibility of reduction of wall thicknesses between 15 to 35% compared with other candidate materials. As an example, Table 1 gives the results of calculation of wall thickness according to the steel grade for pipes for an application at 370 bar 320 °C with an inner diameter of 480 mm. This clearly shows the potential reduction in wall thickness of P36 in comparison to other grades. Using thinner pipes allows in time and cost savings in material, welding /PWHT operations and reduces the weight of structures, column and supporting hangers.

Table 1   Comparison of wall thickness requirement for different grade steels

2.0     Steel Properties 

2.1       Applicable Standards & Product Forms

Various applicable standards and their product forms are given below.
ASTM A 182 Grade F36 Forgings
ASTM A 335 Grade P36 Seamless pipes
ASTM A 213 Grade T36 Seamless Tubes.
BSEN 10216-2 Grade 15NiCuMoNb5-6-4 Seamless Steel tubes

2.2       Chemical Composition

The chemical composition of WB36 according to ASME is given in Table 2 along with SA 106 Gr C for comparison. Grade C belongs to the group of carbon manganese steels. The composition of WB36 differs in nickel, copper and niobium content.

2.3       Mechanical properties

The pipe material has been included in ASTM standard “Specification for seamless ferritic alloy steel pipe for high temperature service” under A335 P36. The specified properties for the pipe material as per ASTM (2005) are given in Table 3.  WB36 composition has been optimized to achieve high yield and tensile strength values. The strengthening effect is obtained by means of grain refinement through the addition of Nb. A second effect is partial hardening by Cu- precipitates. During the development of WB36, it was found that a Cu/Ni-roughly 0.5 is needed in order to avoid hot shortness during hot forming.

Table 2 Chemical requirements of WB 36 (%)

Table 3   Specified mechanical properties of WB36 steel

* Specified minimum

3.0     Experimental Set Up

3.1       Joints
Pipe butt joints of diameter 323.9 mm and wall thickness 40 mm were made with Gas Tungsten Arc Welding process for root welding. Diameter of filler rod and 2% ceriated tungsten rods are 2.4 mm. Since P36 and the filler used are of low alloy, purging of root was not carried out. Then the balance thickness was completed by Shielded Metal Arc Welding.  The electrodes used are diameters 2.5 and 3.2 mm and 4 mm. The electrodes are backed at 300- 350°C before welding. The recommendations of pipe manufacturer for preheat temperatures and interpass temperatures are given in table 4.
Preheat temperature of 120 °C was used to avoid the risk of hydrogen cracking after welding. Inter pass temperature was maintained within 300°C. BS2633 recommendations are similar to V&M recommendations. PWHT shall be applied directly from welding /preheat temperature without any need for cooling to room temperature. The pipe butt joints were subjected to radiographic test to assess the soundness of the weld
Table 4

3.2       Welding Consumables
Welding consumables are selected to nearly match the base metal composition.Weld metals are with Mn-Mo or Ni-Mo alloyed types. For GTAW welding, Union I Mo brand (AWS A 5.28 ER 80SG) was used, which is 0.5 Mo type as per the recommendations of pipe manufacturer. M/s Metrode recommends the use of Mn Mo brand (AWS A 5.28 ER 80SD2/ER 90S-D2) for GTAW.  Welding parameters are voltage 14-16 and current 90-120 amps. Argon is used as shielding gas with gas cup size of 6 mm with a flow rate of 12 liters per minute. For SMAW AWS A 5.5 E 9018 G electrodes (Brand name Phoenix SH Schwarz 3K Ni) were used. Welding current of 80-100 amperes for 2.5 mm diameter electrodes,90-140 amperes for 3.2 mm diameter electrodes and 140-180 amperes for 4.0 mm diameter electrodes were used. Weld Metal deposit analysis are reported in table 5 and meets the requirement of A number 2.
Table 5

Welding consumables of both GTAW and SMAW are tested as per the applicable AWS classifications and the test results are given in table 6.  Both the weld deposits exceed the minimum requirements of base metal P36/WB36.
Table 6

4.0     PWHT
Post weld heat treatment has to be carried out to relieve the internal stresses caused by welding and the tempering of heat affected zone is to be ensured.  PWHT was carried out in a gas fired furnace with necessary thermocouples. As per ASME B 31.1 for ASTM A335 P36 and ASTM A182 F36 materials, post weld heat treatment is mandatory under all conditions. For class 1 material the recommended range is 595 – 650°C and for class 2 components recommended range is 540 – 620°C. Class 1 components are to be held at PWHT temperature for one hour per 25 mm thickness up to 50 mm thickness with minimum soaking time of 15 minutes. For over 50 mm thickness, add 15 minutes minimum for each additional 25 mm thickness. Class 2 components are to be held at PWHT temperature for one hour per 25 mm thickness with minimum soaking time of 30 minutes. The rate of heating and rate of cooling rate above 315°C was maintained at 115 °C/h.
5.0     Test Results
The pipe weld joints were tested as required in ASME Sec IX. The tensile test as per QW 150 and bend test as per QW 180 were carried out. Location of failure in tensile test specimen is parent metal and the ultimate unit stress reported are 675 and 683 MPa. All the four side bend specimens have passed the 4t,180° guided bend test. In addition to mandatory tests weld hardness measurements were taken. The readings in Hv are given in table 7.
Table 7

4.1       Weld metal toughness
ASME base material does not mention the required toughness for P36 but the EN standard specifies minimum 27 joules at room temperature. Adequate toughness is required for weld metal during the pressure testing of the boilers. The weld metal has average toughness values of 120 joules.
4.2       Elevated Temperature tensile strengths
P36 weld metals are having significant importance due to high temperature applications. The hot tensile properties of the weld metal do not necessarily provide information on the joint properties but they are important because design is based on yield strength and not on creep properties.  The all weld tensile tests for SMAW weld metal were reported for three different test temperatures and the results are given in table 8.

Table 8

Summary
The material P36 has specific chemical composition and mechanical properties at room temperature and elevated temperatures. Welding consumables and the welding parameters are qualified to match the base metal requirements. The test results met the ASME requirements and procedure qualification record was qualified, based on the PQR results Welding Procedure is qualified for P36 material for 5-80 mm thickness by GTAW and SMAW processes.
References

1. Vallourec & Mannesmann Tubes “The WB36 Book (15NiCuMo)”, 2002.
2. ASME B31.1 -2016 Power Piping
3. ASTM A335/A335M-06 “Standard specification for seamless ferritic alloy-steel pipe for high temperature service.”
4. ASME Boiler & Pressure Vessel Code IX-2017.
5. Welding Studies on WB36 for Feed Water Piping-Sathish Kumar R, Dr. T Ramesh, and  K Asokkumar
6. BS EN 10216-2:2002 “Seamless tubes for pressure purposes – Technical delivery conditions Part 2.Non-alloy and alloy steel tubes with specified elevated temperature properties.”
7. Technical profiles October 2009 by Metrode Products Limited.