Influence of post weld heat treatment for weld joint of P355GH

From steel designed to work under pressure and exposed to high temperature apart from the good weldability, good mechanical properties are required. The guidelines set by the regulations require post welding heat treatment above 35mm thick. An important factor affecting the microstructure and properties of the joint made of thick-walled elements is heat treatment after welding. All welding operations must be properly planned before performing welding work. Welding procedure specification (WPS) is a document describing these operations, it is essential for proper determining of basics in planning welding operations and quality control in welding. The purpose of this paper is to compare the properties of joints made by 121 welding method in combination with and without post welding heat treatment.


Introduction
From steel designed to work under pressure and exposed to high temperature apart from the good weldability, good mechanical properties are required. The guidelines set by the regulations require post welding heat treatment above 35mm thick. An important factor affecting the microstructure and properties of the joint made of thick-walled elements is heat treatment after welding. All welding operations must be properly planned before performing welding work. Welding procedure specification (WPS) is a document describing these operations, it is essential for proper determining of basics in planning welding operations and quality control in welding. The purpose of this paper is to compare the properties of joints made by 135+121 welding method in combination with and without post welding heat treatment. The MAG welding process (135) (Metal Active Gas) involves melting an edge of joined materials and a material of a consumable electrode by the heat of an electric arc in the shielding gases or gas mixtures [1]. The heat source in the MAG welding method is a welding arc glowing in the gas atmosphere between a consumable electrode and the weld material [2,3]. This method fully meets the requirements which are given to thick walled joints both in terms of technology and efficiency [5][6][7][8]. This welding process is currently. The SAW welding process (121) (Submerged arc welding) with high heat input has been used to weld thick plates in order to reduce the number of passes and consequent increase of productivity in the shipbuilding and oil industries. Additional characteristics of SAW such as high penetration and relative ease to produce welds with good finishing and without discontinuities contribute to this process has being widely applied. However, the use of very high heat inputs leads to the formation of a large melting pool and large amount of liquid metal, and it submits the weld region to long-term thermal cycles with low cooling rates. This scenario contributes to the formation of thick solidification structures and, at the end of the cooling, results in a microstructure consisting mainly of grain boundary ferrite with low mechanical strength and large grain size [9].

Materials used for testing
P355GH steel was used as the basic material. It is a steel often used as a material for heat exchanger coats because its strength and plastic properties ensure no brittle fracture in the operating temperature range. According to the PN-CR ISO 15608 standard [11], P355GH steel belongs to group 1.2. It is a non-alloy quality steel, working under pressure and high temperature (up to about 400 ° C), whose yield strength for the smallest dimension range is 355 MPa. The chemical composition of the P355GH (Number 1.0473) steel is given in Table 1, mechanical properties in the standardized state are shown in Table 2.

Examinations of welding technology
In order to evaluate the welding technology, numerous tests were carried out on test joints (1 and 2), adopting acceptance criteria included in the qualification standard for the specification of the initial welding procedure by testing the welding technology in accordance with the PN-EN ISO 15614-1 standard [12]. The tests were carried out on test joints (1 and 2) prepared in accordance with the initial preliminary welding procedure specification (pWPS) for MAG method (131) and SAW method (121) with heat treatment -No. 1. Combinations of MAG (131) and SAW methods (121) without heat treatment -No. 2.

Sample welded joint
The welded test joint was prepared in accordance with the requirements of the PN-EN ISO 15614-1 [12] standard. The test joints used for the test were made of 300 x 35 mm and 30 mm steel sheets of P355GH type steel. Both Joints (1 and 2) were made in the low position PA using the X bevel ( Figure 1). The detailed data described in the specification of the initial welding procedure is shown in Figure 1 and Table 3.  In the welding process for method 135, a manganese-silicon wire with a diameter of 1.2 mm was used. This wire is designed for fine-grained carbon-manganese steels and lowcarbon construction steels: G3Si1 according to EN ISO 14341-A (OK Autrod 12.51). An M21 gas mixture (82% Ar + 18% CO2) was used methods 121 wire with a diameter of 3 mm. For the method 121, a copper wire with the addition of manganese with a diameter of 3 mm was used. This wire is intended for welding structural steel of high and medium strength. After the welding process, the joint 1 was heat treated after welding. A stress relief annealing was performed at 620 ° C for 2 hours. The essence of thermal stress relief is to lower the yield point or creep at elevated temperatures, and thus to allow plastic deformations to take place in those areas where the internal stress exceeds these limits. Relaxation annealing is aimed at: 1) invoking optimal relaxation of remaining (residual) stresses, 2) to restore ductility in brittle zones after welding.

Examinations of a sample welded joint
Samples for destructive testing were taken after obtaining positive results of successively done NDT on the sample joint including the tensile test, bending, impact strength, macroscopic examination, and hardness distribution. Appropriate standards of examination were agreed to do non-destructive testing as well as an assessment of joints was adopted, which are presented in table 4. Samples for destructive testing were collected in accordance with PN EN ISO 15614-1 [12].

Results
After the visual inspection in samples 1 and 2 no welding incompatibilities were found. No floods, discontinuities, etc. were observed. The weld geometry was correct. After the ultrasound and penetration tests, no welding incompatibilities were found. The transverse stretching test, as well as the longitudinal stretching and the bending test for both joints (1 and 2), gave positive results. The tearing of the samples in the longitudinal stretching test occurred outside the weld and the tensile strength value was above the lower limit of the basic material value (table 5).  After bending tests on U, no welding inconsistencies were found in the samples. There were no tears affecting the properties of the connector (table 7).     I  II  III with a coarse structure is shown in Figures 4b and 5b. Fusion lines are shown in figures 4c and 5c. Welding microstructure consisting of Widmanstätten ferrite and fine-plate ferrite is shown in Figures 4d and 5d.