Weld procedural effect on the performance of iron based hardfacing deposits on cast iron substrate

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Abstract

Two commercial and four formulated iron based high Cr high C type hardfacing electrodes were deposited on gray cast iron plate (ASTM grade 2500) using various welding procedures such as preheat and without preheat, single and double hardfacing layer as well as buffer or without buffer layer. The effect of welding procedural variation upon the cracking sensitivity and performance of interface between substrate cast iron and deposited layers were studied. Since there is no standard method for evaluating the performance of such interface, shear strength values at their interfaces have been considered and compared among different weld deposits made with different procedures. The results show that crack length per unit area of the deposits is affected by the welding procedure and Cr/C ratio of the hardfacing electrodes used. Though high nickel buffer electrodes attributed highest bond strength, low hydrogen type electrodes could also be used at comparable bond strength with better cost effectiveness.

Introduction

Improved performance and prolonged service life are basic qualifications that have recently come to be required for industrial plants and machineries. This is particularly true for large installations containing components of which wear rates greatly affect the overall plant performance and service life. One such component used in one of the thermal power plant in west Bengal, India, is top bearing plate (TBP) which is made of ASTM grade 2500 cast iron. Fe–Cr–C based hardfacing deposits are typically applied to a wide variety of worn out surfaces of the TBP. These hardfacing deposits usually have one or two layers so that the effect of dilution is significant. Since both the substrate cast iron and hardfacing deposits are inherently brittle, cracking can occur as a result of welding contraction strain. Such cracking does not necessarily significantly reduce the service wear life of the component, and indeed is sometimes seen as an advantage in reducing residual stress levels [1]. Nonetheless, in many instances cracking is undesirable, whether to obtain a scaling surface or to avoid spalling, for example, and a requirement exists for deposition of crack-free, and high hardness surfacing.

In principle, cracking can arise either in the solid state because of low tensile ductility or during solidification. The latter mechanism of cracking can normally be overcome by reducing travel speed with attention to arc extinction procedure to avoid crater cracking. The former crack type represents a rather more intractable problem in hardfacing alloys. Essentially, the incidence of cracking can be related to the tensile ductility of the deposit, and hence to its composition and hardness. While cracking can, in principle, be avoided by selection of an alternative consumable composition, this will generally involve a reduction in deposit hardness which may be unacceptable in terms of service properties. Where such a material change is inapplicable, the most common preventive measure is to apply preheat [2], [3], on the basis that the cooling rate after welding can be reduced with a concomitant reduction in the differential contraction strain between the overlay and the substrate. Furthermore, buffer layers are often employed between substrate and the hardfacing deposits in order to minimize dilution and contraction strain. In case of cast iron, high nickel electrode is commonly used as buffer layer. The use of nickel-base buffer layer significantly decreases the extent of carbon migration from the cast iron into the hardfacing layer because of low diffusivity of carbon in nickel-base alloys. The only drawback of nickel-base buffers is their high cost. Indeed, there is a growing realization that hardfacing alloys cannot be deposited on cast iron without prior deposition of nickel-base buffer. Furthermore, the role of nickel-base buffer in contributing the bond between substrate and the hardfacing deposits is also not clear. However, few qualitative data have been published [4] on the effects of welding procedures on the cracking behaviors of weld deposited hardfacing as well as performance of interface between substrate and the overlay using different types of buffer layers or even without buffer layer.

In view of the above, the present investigation has been initiated to study systematically the weld procedural effects on cracking sensitivity of weld deposited hardfacing alloys on cast iron as well as performance of the interface between substrate cast iron and hardfacing deposits. This will ultimately facilitate in affording guidance on welding procedure to avoid cracking as well as to select suitable buffer layer based on their bond strengths between substrate and hardfacing deposits.

Section snippets

Materials

The substrate material was sand mould casted gray cast iron plate (ASTM grade 2500) with dimensions of 90 mm × 85 mm × 20 mm. Hardfacing were done upon these gray cast iron substrate plates using six different (two commercial and four formulated) high carbon–high chromium hardfacing alloys. The commercial hardfacing consumables, in the form of solid wire coated electrodes, were used as per the direction of the electrode manufacturer. The formulated electrodes were used, in the form of tubular wire

Results and discussion

Main objective of the present investigation was to study the effect of welding procedure such as hardfacing deposits with different types of buffer and without buffer, single and double hardface layer, preheat and without preheat upon the cracking sensitivity and performance of interface between substrate cast iron and hardfacing deposits. Finally, to check up any influence of these performance on abrasive wear properties. To explore the performance of such interface, i.e. between substrate

Conclusions

The following conclusions may be drawn from the above study:

  • 1.

    An experimental investigation of iron based hardfacing deposits on cast iron substrate with different weld procedures has shown significant differences on their performance in terms of susceptibility to surface cracking and bond strength between substrate and buffer layers or hardfacing deposits (in case of without buffer).

  • 2.

    Susceptibility to surface cracking of deposits quantified as crack length per unit area is affected by preheat,

References (10)

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