1. What is hardfacing?
Metal parts often fail their intended use not because they fracture, but because they wear by abrasion, impact, metal-to-metal contact or some other form of wear, which causes them to lose dimension and functionality. Hardfacing, also known as hard surfacing, is the application of a build-up or wear-resistant weld metal onto a part's surface by means of welding to extend the life of the part. The weld metal may be applied as a solid surface or in a pattern, such as a waffle pattern, herringbone or dot pattern, etc.
Hardfacing is becoming increasingly important for many industries to protect equipment that is exposed to wear and abrasion. hardfacing is used in mining, dredging, recycling, farming, railroads, earthmoving and construction, cement, logging, power generation, oil and gas drilling, as well as steel making and forging. Extending the life of wear parts will save thousands of dollars and improve productivity. Hardfacing can be used to recondition parts that have already been exposed to wear and have lost their useful life, or used in the manufacture of new parts to improve their life before they are put into service.
There are three main types of hardfacing applications:
- Build-up or Rebuilding
- Hardfacing or Overlay
- A Combination of Build-up and Overlay
Reasons for Hardfacing Companies use hardfacing products for the following reasons:
• Reduced costs – hardfacing a worn metal part to a like new condition is usually 25–75% of the cost of a replacement part.
• Prolonged equipment life – hardfacing extends life 30%–300%, depending upon application, as compared to that of a non surfaced part.
• Reduced downtime – because parts last longer, fewer shutdowns are required to replace them.
• Reduced inventory of spare parts – there is no need to keep numerous spare parts when worn parts can be rebuilt.
Uses for Hardfacing
There are two main areas where hardfacing is used:
• Rebuilding worn metal parts to their original dimensions. This is accomplished with build-up or with build-up and overlay. In both cases, the rebuilt part is usually superior to the original part. Worn parts that remain basically sound can be surfaced again and again provided that correct procedures are used.
• Protecting new metal parts against the loss of metal. Hardfacing overlay is used on both new and/or original equipment where the part is most susceptible to wear.
The higher alloy overlay offers much better wear resistance than that of the original base material. This usually increases the work life of the component up to two or more times that of a part which is not surfaced. Although the added hardfacing material may add to the price of the equipment, usually a less expensive base material may be used.
What base metals can be hardfaced?
Carbon and low-alloy steels with carbon contents of less than 1 percent can be hardfaced. Medium carbon and low-alloy steels are very common since they provide higher strength than mild steels and better abrasion resistance. High carbon alloys may require a special buffer layer.
The following base metals can be hardfaced:
- Stainless steels
- Manganese steels
- Carbon and alloy steels
- Cast irons
- Nickel-base alloys
- Copper-base alloys
Carbon and low-alloy steels are strongly magnetic and can easily be distinguished from austenitic manganese steel which is non-magnetic. There are many low-alloy and higher carbon steels that are used for manufacturing equipment and spare parts, especially equipment that requires higher strength and abrasion resistance. They are not easily differentiated but should be identified in order to determine proper preheat and post-heat temperature. As the alloy content increases, the need for preheat and post-heat becomes more critical.
Dilution In Hardfacing
Dilution is defined as the change in chemical composition of a welding filler metal caused by the admixture of the base metal or previous weld metal in the weld bead. It is measured as the ratio between the base metal to the filler metal in the weld deposit (see below). That means the dilution percentage is the amount of base metal (or previous weld metal) that ends up in weld deposit. When joining two metals, the strength of the joint is determined by the amount of penetration or dilution. In hardfacing there is no need for high penetration, as only a bond between the weld deposit and base metal is required. Since the chemical composition and properties of hardfacing overlays are usually quite different from the base metal, too much dilution can be detrimental. Hardfacing overlay alloys are carefully formulated to furnish specific wear characteristics in the minimum amount of welding passes, so as dilution increases, the wear characteristics decrease.