ARC MAGAZINE

OXYFUEL COBALT OVERLAY ON STAINLESS STEEL

Oxy-acetylene welding is one of the oldest forms of welding. It combines a fuel gas, acetylene, with oxygen to raise the flame to a very high temperature – up to 6,300° F, more than adequate to melt most metals. Oxy-acetylene (or OAW) welding was developed in the early 1900s - only slightly after arc welding was developed – and it quickly became a popular process in manufacturing and fabrication because it enabled workers with minimal training to create high-quality welds. It was also an extremely mobile process, as it required no electricity or flux covered electrodes.

Proper OAW welding requires the correct fuel-to-air mixture in order to produce adequate flame temperature. Most OFW welds are created by the neutral flame, which includes an equal balance of acetylene and oxygen. A reducing flame (or carburizing flame) contains more fuel than oxygen, while an oxidizing flame contains more oxygen than fuel. Different torch sizes and different volumes of gas allow for the welding of different metal thicknesses.

Although not as popular as it once was, oxy-acetylene welding is still used in various industries, including petrochemical, nuclear, polymer, and general maintenance and repair. The petrochemical, nuclear, and polymer industries use the oxyacetylene process to overlay high-wear or corrosive-resistant weld metals on surfaces that are used in severe or critical service conditions. The overlays are often used for wear components in high-temperature valves. The maintenance and repair industry uses the process to fix cast iron products with braze-welding. It’s not the fastest method of welding, but the quality of the end result justifies the extra time.

In this demonstration of oxy-acetylene welding, I’m overlaying stainless steel with hardfacing and corrosion resistant material known as Weartech 6, or AWS A5.21 ERCoCr-A. Weartech 6 is a cobalt alloy. The oxy-acetylene process is beneficial in that it produces an adequate layer of abrasion- and corrosion resistant surface overlay with a typical dilution of 5% or less at the weld interface – much lower than GTAW and GMAW. The low dilution will allow for maximum performance of the overlay.

Before welding, preparation is important. Remove all scale with a sanding disk or weld on a freshly machined surface. Acetone or some other non-chlorinated degreasing agent will remove any excess oils. After you’ve selected the correct torch size, ignite the acetylene and add oxygen to the fuel mixture until flame feather is about three times longer than the inner cone. Hold the torch to where the inner cone is almost contacting the base metal. You’ll start to see it heat up and turn red.

Once the base metal surface has reached melting temperature, pull away slightly and begin adding the overlay. Use a daubing method by balancing the heat between the base metal and filler metal. This will maintain molten surfaces and also allow for fusion between the two materials. After the weld is complete, usually after a minimum of 2 layers, pull the flame away slowly to allow for filler metal solidification and to reduce contamination by atmospheric air. Carefully place the weldment in a bucket or tub filled with vermiculite or sand to allow for slow and controlled cooling and to reduce the chance of cracks.

Even though this method of overlay is not very common, it does produce extremely desirable results for the industries in which it’s still used. Always remember to select the correct Harris torch size, and ensure that you have adequate acetylene volume prior to welding. Base materials need to be extremely clean and flame adjustment is critical to produce an acceptable weld. Taking the extra time to focus on the small details will pay dividends in the end.

Tools

Soapstone to mark area to be preheated

Temp stick to measure the temperature of the steel prior to welding

Oxy-propane torch (with striker)

Personal Protective Equipment

Heat-Resistant Welding Gloves
Safety Glasses
Welding Coat

PREHEAT SAFELY

We always want to make sure we’re working safely when welding and working with torches. Refer to ANSI Z49.1: “Safety in Welding, Cutting, and Allied Processes,” which includes heating.

Make sure to wear the proper safety gear, including flame-retardant clothing, gloves and proper safety glasses. Make sure your jeans or pants are not frayed.

For proper torch setup and application, refer to the manufacturer’s guidelines. Make sure to have the correct check valves on the torch, flash arresters back at the regulator, hoses in good condition, and regulators set and operating properly.