In many large industrial plants, such as, power generating plants, pulp and paper mills and refineries, most major plant equipment, such as boilers and pressure vessels, is manufactured from carbon steels or low alloy steels for pressure containment. These components are generally designed and constructed based on strength requirements following codes and standards, such as ASME Codes. Although most of these components have corrosion allowance build into their initial wall thickness, wastage rates due to corrosion can be excessive for carbon steels or low alloy steels. Thus, boilers or vessels, in many cases, could not operate economically without some sort of surface protection against corrosion or corrosion/erosion. One cost-effective, engineering solution is to use a surface protection layer to protect carbon steel (or low alloy steel) boiler tubes or vessels against corrosion attack. This approach allows the substrate material (i.e., carbon steels or low alloy steels) to provide strength requirements to meet codes and standards for pressure containment while relying on the surface protection layer to protect the equipment against corrosion, thus, allowing the equipment to operate in a cost-effective manner.

Weld overlay had been used in the past as a temporary, “band-aid” type repair in the field until a somewhat permanent fix could be developed to address the corrosion problem. Thanks to advances in automatic welding system and process control, it is now possible to overlay a large area of major equipment, such as, the waterwall of a boiler or the internal diameter of a reactor vessel, with a
corrosion-resistant alloy to significantly minimize or essentially eliminate the corrosion problem.

Modern weld overlay has now become a long-term fix to fireside corrosion problems for boiler tubes in waste-to-energy boilers, coal-fired boilers and recovery boilers, and to corrosion problems due to processing streams in reactor vessels in pulp mills, refineries and petrochemical plants.

Major corrosion problems in waste-to-energy boilers, coal-fired boilers, kraft recovery boilers, digesters in pulp mills, and refinery vessels are described. Modern weld overlays applied by advanced automatic overlay welding machines are discussed. The merits for using these modern weld overlays for corrosion protection for large industrial equipment are also discussed. The use of modern weld overlays has been proven to provide long-term corrosion protection for the aforementioned systems. Overlays of nickel-base alloy 625 has been extremely successful to minimize the chloride corrosion attack on waterwalls and superheaters in waste-to-energy boilers. Both type 309 SS and alloy 625 overlays have been very successful in reducing or essentially eliminating sulfidation attack on the waterwalls of coal-fired boilers equipped with low NOx burners. Also successful in mitigating corrosion problems in kraft recovery boilers are alloy 625 for floor tube and membrane overlays and 309 SS and 625 overlays for corrosion protection in lower furnace waterwalls. Kraft digesters have relied on 309 SS and 312 SS weld overlays for corrosion protection. Many vessels, towers and columns are weld overlaid with austenitic alloys, such as, 309L, 317L, alloy 82, etc., in petroleum refineries. The weld overlay approach has also been used as an effective means to manage corrosion and wear problems for vessels and reactors in petrochemical/chemical processing, for waste heat boilers in mineral ore roasting operations, and other industrial systems.

Machining of austenitic stainless steels

When appropriate consideration is given to the special characteristics of the high-performance

stainless steels, they can be machined successfully by all the methods commonly used to machine the standard stainless steel and nickel alloys. Compared to the 300-series austenitic grades, the high-performance stainless steels have:

1. higher room temperature and elevated temperature strength

2. higher work hardening rates

3. similar galling characteristics

4. extremely low sulphur contents.

As a result, machining will be more difficult than with the standard grades, and careful attention must be given to detail to ensure success.

The basic machining principles that apply to the standard stainless steel grades and nickel-base alloys are a good starting point for machining the high-performance stainless steels. These include sharp tools, rigid setups, positive feeds, adequate depths of cut, positive cutting geometries where possible, and quality tooling and coolant designed for stainless steels. Feed rate and depth of cut are very important if there will be a subsequent finishing operation because prior surface work hardening effects must be removed as much as possible before attempting shallower finishing passes. Finishing passes should be as deep as possible to cut below the work hardened surface layer. High cutting tool toughness is helpful because of the high strength of the stainless steel. High machine power is also important because of

the high strength and high work hardening behaviour of these stainless steels.

Of the three stainless steel families, the austenitic stainless steels are the most difficult to machine. These grades, especially the more highly alloyed subgroups, have machining characteristics similar to the corrosion resistant nickel-base grades in the solution annealed condition. The ferritic grades are the easiest to machine. Machining parameters that would usually be used for Type 316 stainless steel can provide a starting point for working with the high-performance ferritic stainless steels. The duplex grades are about halfway between Type 316 and the high-performance austenitic grades.