Alloy Steel in High Temperature Services
Welding at high temperatures demands that the weldment be made of alloy steel because of two property limitations of iron and carbon steel,
which grow weaker as temperature rises. These are a decrease in strength and an increase in corrosion and oxidation.
Chromium and moiychrom steel, also known as chrom-moly, were originally developed for elevated temperature services. They have been extensively used in power plants, pressure vessel, heat exchangers, and other applications where very high temperatures are involved. Moiychrom steel is used because it maintains strength at high temperatures and also resists creep, which means that it does not stretch or deform under load after long periods of use with high pressure and temperatures. Carbon steels, on the other hand, tend to stretch at high temperature service; they will become brittle in time.
A number of compositions have become popular. The molychrome steel is hardenable; therefore, the welding procedure must includes postheat treatment. The preheating temperature depends on the specific steel composition, thickness of material, and compacted joint designs.
Preheating and postheat treatment are important part of any welding procedure. But welding in itself brings out certain important factors. For example, moly chrome has high strength and creep resistance. Therefore, welds made on this alloy should not contain any defects such as undercut, poor restart, or humps along the fusion line and edges, where a smooth transition into the edges of the base metal is needed. In addition, after the weld is completed, the slag covering on the finish weld should be properly cleaned by using a wire brush, preferably stainless steel. The reason for this is that chromium in steel combines with oxygen to develop an oxide on the surface of the weld. As a protective coating to prevent further oxidation and scaling, a completed weld must be clean of its slag and wire brushed so that, within a given period of time, the oxygen in the atmosphere will react with the weld metal and the surrounding area to develop such a protection.
A critical aspect of the SMAW process is the proper angle of the electrode in relation to the work. The electrode angle should never be more than 10 to 15 degrees off the perpendicular tangent to the pipe surface as it travels around the pipe. It is this requirement among all others that make the art of pipe welding so difficult to master. If the electrode angle is excessive, the arc energy is not focused properly on the work. In addition, and most importantly in the welding of the alloy steels, excessive electrode angle causes erratic burning of the flux coating which is often the cause of surface porosity and difficulty in maintaining the proper arc length. This occurs because as the electrode bums, an extreme angle to the work causes unconsummed coating to hang over the tip of the electrode much like a finger nail and actually drop off into the weld puddle. As 50 to 70% of the weight of an alloy electrode consists of supplementary alloying material contained in the coating, flakes of the electrode coating falling into the weld puddle can remain as permanent slag inclusions rather than enhancing the weld metal as it was intended to do.