DEVELOPMENT OF BASIC COATED HP40Nb ELECTRODE

Published on 11 October 2022

  • book8 min

DEVELOPMENT OF BASIC COATED HP40Nb ELECTRODE SUITABLE FOR HEAT AND CORROSION RESISTANCE CASTINGS.



1. Introduction

In petrochemical plants the welding consumable used in fabrication of equipments grade such Alloy 20-32Nb (ASTM A351/ A351M-05) must have good corrosion resistance, high heat resistance and improved hot tensile strength along with good weldability properties to enable onsite repairing. The petrochemical industry uses heat-resistant austenitic stainless steels (Series H) for the fabrication of columns for water vapor reform furnaces and pyrolysis furnaces. These furnaces are used for converting water vapour into a hydrogen-rich gas which is then supplied to water treatment plants to improve fuel quality or used in other applications. Components used in petrochemical industry must continue in function for long periods of time under severe environment. In view of this, a suitable SMAW welding consumable with improved mechanical properties have been developed in-house for fabricating HP-Nb modified (ASTM A297) alloy or similar materials such as such as HT40, IN519, HK40. Continuous research from many decades on these alloys made it possible to achieve increased mechanical strength at high temperatures which has prolonged their service life in sever environment Among various developments for these steel is the addition of carbide forming alloy elements such as Nb, Ti, Zr, W, rare earth mixtures and more recently, the addition of Y that are stable at high temperatures. The high carbon content of these steels, which is necessary for their high-temperature strength, gives these alloys little mechanical conformability by conventional processes. It is therefore necessary to characterize the microstructures of these materials to gain an understanding of the behavior, morphology and composition of precipitated carbides in order to relate these characteristics with the material properties. 

Table 1 - Specification to be Met as per Customers’ Requirement

                   

2. Design of Product

Developmental work has been designed with performance of the weld to meet the requirement of strength, hardness and toughness. Several trials have been designed by keeping the following parameters in mind. (i) Variation of chemistry within the specification limits. (ii) Selection of high purity raw materials in flux formulation. (iii) Performance characteristics of the product including slag detachability. (iv) Control on S, P, Sn, Sb, & As to resist cracks. (v) Selection of suitable core wire & its purity level. 

3. Experimental Work

Several trials have been taken and established the weld metal chemistry to meet customer’s requirement. Tramp elements are restricted to a very low level and further reduced the Mn & Si content of the weld metal. The weld metal composition meeting the requirement is confirmed by optical emission spectroscopy. The all weld test coupons were prepared by these electrodes. These test coupons are taken up for characterization and mechanical property evaluations.

3.1 Preparation of Test Coupons

A small section of IS 2062 material with buttering is used as a base material for joining purpose. A single V groove having 22.5 degree bevel angle is made on the base plate. The all weld joint is supported with a backing strip made of mild steel having dimension 155x35×6 mm same has been buttered. In this study we have followed customer’s guidelines for preparation of weld coupons. The welding parameters such as heat input, preheat & interpass temperatures are controlled to get good quality welds. The WPS followed are shown in the Table-2. The all weld test specimens are machined out after x-ray from the each weld joint assembly and are subjected to various tests including metallographic & mechanical testing.

Table 2 Details of the Welding Parameters Used in Present Study

                      

                      

3.2 Penetrant and Radiography Test

The weld deposits are analyzed with Dye penetration between the passes and finally sent for X-ray for the evaluation of any presence of crack and inclusions.

3.3 Tensile Test

The tensile property of the weld specimen is analyzed using AMSLER Universal Tensile Testing Machine with a load capacity of 200 KN. The tensile measurements have been conducted at room temperature (RT). The round specimens of diameter 6.25 mm and gauge length of 25 mm used for tensile testing prepared as per the AWS B4.0 standard. 

                     

                   

The tensile data is analyzed to estimate the yield strength (YS), ultimate tensile strength (UTS), total elongation (TE) and RA (%). The tensile test data as shown in Table-3 are analyzed to evaluate the ultimate tensile strength (UTS), yield strength (YS), %elongation (EI) and reduction in area.

Table 3 - Detail of All Weld Tensile Test Results

                    

3.4 Hardness Test

A square base pyramid shaped diamond is used for testing in the Vickers scale. Typically loads are very light, ranging from a few grams to one or several kilograms. Hardness measurements were taken on the transverse cross section of weld which covers weld region. Vickers hardness tester was used with 10kg of indentation load, dwell time of 20 sec at room temperature. Initially the surface of the weld samples were polished up to 1200 grit fineness followed by cloth polishing through 0.05μm grain size alumina powder and finally etched. Readings of weld metal is shown in Table-4.

Table 4 Vickers hardness test results (HV30) 

                   

3.5 Microstructure Analysis

In the as-welded condition the weld metal consists of austenite with eutectic and secondary carbide. Alloy HP40Nb is not prone to sigma phase Embrittlement and the presence of eutectic and secondary carbides in austenitic matrix provides excellent hot strength and creep resistance in the typical service temperature range. High levels of Cr and Ni provide good resistance to oxidation and carburisation.

                     

4. Discussion

In order to develop a suitable consumable for high heat resistance, improved strength and good corrosion resistance, it is vital to develop a weld metal chemistry that meets all requirements. According to the necessities, consumable variables such as electrode size, flux and composition have been established by continuous research and laboratory testing. Careful optimization of the composition of weld metal and process parameters such as current, voltage, speed yields good mechanical properties and sound quality weld deposit. Evaluated testing results of weld metal shows that The SMAW electrode developed in house has meet all the requirements of as per customers’ specifications. The radiography and liquid penetrant test have confirmed sound quality weld metal deposit. Hence the careful optimization of weld metal chemical composition of welding consumable and maintenance of weld metal quality makes this product applicable for petrochemical industry applications as well as various challenging structural applications.

5. Conclusion 

  • Suitable SMAW process consumables were developed for welding ASTM A297 - HP40Cb, HP and HP40 alloys or similar composition alloys. 
  • Consumables with lesser amount of P, S, As, Sn, and Sb yields good strength & heat resistance. 





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