Thermal Fatigue Characteristics of Pta Hardfaced Steels

Abstract

Many types of hard material are coated on the surface to improve their wear resistance. Addition of vanadium carbide to Co based alloys (stellite no. 21) as a hard material powder is one of the ways to improve the wear resistance characteristics of the surface layer. The plasma transfer arc (PTA) welding process was introduced as a coating technology for elevated temperature surface modification. This process has recently generated interest in the surface modification field owing to its operability, low initial cost of equipment, high deposition rate, and small dilution rate. Coated layers produced by PTA considerably improve the hardness and wear resistance of surface layers for elevated temperature applications. Vanadium carbide (VC) addition into stellite powder showed a significant improvement in wear resistance. However, alloys containing VC showed pronounced sensitivity to hot cracking under repeated heating and cooling environments. This study clarifies the cause of thermal fatigue cracking in Co based alloy deposits with VC powder additions. Cracks result from the difference in thermal expansion coefficient between the matrix and the carbides. Cracks initiate in the central part of the surface region and grow in a perpendicular direction towards the surface. The tendency for thermal fatigue crack initiation seems to increase with increasing carbide volume fraction and decrease as the volume fraction of the dendritic region decreases.

Effect of cryogenic treatment on tribological behaviour of TiC composite coatings

“…Hard metallic carbide powders such as TiC [4,5], Cr 3 C 2 [6], B 4 C [7], and WC [8] are commonly used in composite coatings deposited on metals by this method. The PTA coating process has benefits, for instance an excellent bonding with the substrate, high deposition rate, low cost equipment, good deposition efficiency, and lower heat input [9][10][11].…”

An Improved Wear-resistant PTA Hardfacing: VWC/Stellite 21

“…Many types of hard material are coated on surfaces to improve wear resistance, strength, and corrosion characteristics. Stellite 21 coatings have been developed by various techniques [9][10][11][12][13][14][15][16][17], for example, plasma transferred arc (PTA) welding and high power diode laser (HPDL) cladding. Stellite 21 contains low carbon content so that its hardness and wear resistance are relatively low compared with other wear-resistant Stellite alloys.…”

“…The properties of Stellite 21 surface coating can be improved by the addition of the embedded hard materials. In the PTA process, these hard materials in the weld pool tend to either separate out of the molten material as hard phases or are segregated as foreign bodies from the weld pool [13]. For wear applications, tungsten carbide (WC) with a high melting point (3410 C) and high hardness (2100 kg/mm 2 ) is very attractive.…”