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.

Ni-Cu Alloys

The two main alloys in this system are Monel 400 or alloy 400 and its
age–hardenable version, alloy K-500. Alloy 400 was developed at the beginning
of the twentieth century and, even after approximately 100 years, continues to be
used in the modern-day chemical, petrochemical, marine, refineries, and many
other industries. Alloy 400 containing about 30–33% copper in a nickel matrix
has many similar characteristics of commercially pure nickel, while improving
upon many others. Addition of some iron significantly improves the resistance
to cavitation and erosion in condenser tube applications. The main uses of alloy
400 are under conditions of high flow velocity and erosion as in propeller shafts, propellers, pump-impeller blades, casings, condenser tubes, and heat exchanger tubes. Corrosion rate in moving seawater is generally less than 0.025 mm/year. The alloy can pit in stagnant seawater, however, the rate of attack is considerably less than in commercially pure alloy 200. Due to its high nickel content (approx. 65%) the alloy is generally immune to chloride stress corrosion cracking.
 
The general corrosion resistance of alloy 400 in nonoxidizing mineral acids is
better compared to nickel. However, it suffers from the same weakness of exhibiting very poor corrosion resistance to oxidizing media such as nitric acid, ferric chloride, cupric chloride, wet chlorine, chromic acid, sulfur dioxide, or ammonia.
 
In unaerated dilute hydrochloric and sulfuric acid solution the alloy has useful
resistance up to concentrations of 15% at room temperature and up to 2%
at somewhat higher temperature, not exceeding 50◦C. Due to this specific characteristic, alloy 400 is also used in processes where chlorinated solvents may form hydrochloric acid due to hydrolysis, which would cause failure in standard stainless steel.
 
Alloy 400 possesses good corrosion resistance at ambient temperatures to all
HF concentration in the absence of air. Aerated solutions and higher temperature
increase the corrosion rate. The alloy is susceptible to stress corrosion cracking in moist aerated hydrofluoric or hydrofluorosilic acid vapor. This can be minimized by deaeration of the environments or by stress relieving anneal of the component in question.
 
Neutral and alkaline salt solutions such as chloride, carbonates, sulfates and
acetates have only minor effect even at high concentrations and temperatures up
to boiling. Hence the alloy has found wide use in plants for crystallization of
salts from saturated brine.
 
Alloy K-500 (UNS N05500), the age-hardenable alloy, which contains aluminum and titanium, combines the excellent corrosion resistance features of alloy 400 with the
added benefits of increased strength, hardens, and maintaining its strength up to
600◦C. The alloy has low magnetic permeability and is nonmagnetic to −134◦C.
Some of the typical applications of alloy K-500 are for pumpshafts, impellers,
medical blades and scrapers, oil well drill collars, and other completion tools,
electronic components, springs and valve trains. This alloy is primarily used in
marine and oil and gas industrial applications. In contrast alloy 400 is more versatile, finding many uses in roofs, gutters, and architectural parts on a number of institutional buildings, tubes of boiler feedwater heaters, seawater applications(sheathing, others), HF alkylation process, production and handling of HF acid, and in refining of uranium, distillation, condensation units, and overhead condenser pipes in refineries and petrochemical industries, and many others.

Service Temperatures for Superalloys

The superalloys, as has been noted, consist of alloys of iron-nickel-, nickel-, and cobaltbase that are destined generally for use above about 1000 F (540 C) and below the melting points of the alloys, which usually are at or above about 2200 F (1204 C). Some superalloys also find use in space applications where subzero and cryogenic temperatures are an issue. The bulk of this text presumes that the application will be at elevated temperature.

Wrought nickel- and iron-nickel-base alloys, in general, have temperature limitations of about 1500 F (816 C). Above that temperature, cast alloys generally are used. The majority of superalloys are strengthened by the production of secondary phases (precipitates), and the upper temperature limit for alloy use is governed by the base (nickel- or iron-nickel-base), the volume/type of precipitate, and the form (cast or wrought).

It is commonly understood in the superalloy industry that certain alloy types are used for specific temperatures of application. For example, most wrought nickel- and ironnickel- base superalloys are used only to about 1200 to 1300 F (649 to 704 C). The range of such alloys actually starts below 1000 F (540 C), frequently as low as 800 F (427 C), and the wrought alloys are particularly useful in gas turbines when titanium alloys might be inappropriate. Cast alloys are used across the temperature range but particularly at the highest temperatures, especially as in gas turbine engines.

Superalloys usually are processed to optimize one property in preference to others.
The same composition, if used in cast and wrought state, may have different heat treatments applied to the different product forms. Even when a superalloy is used in the same product form, process treatments may be used to optimize one property over others. For example, an alloy such as Waspaloy was being produced in wrought form for gas turbine disks. By adjustment of processing conditions, principally heat treatment, substantial yield strength improvements (a desirable effect) were achieved in the wrought product at the expense of creep-rupture strength.

Basic Metallurgy of Superalloys

Iron, nickel, and cobalt are generally facecentered cubic (fcc-austenitic) in crystal structure when they are the basis for superalloys. However, the normal room-temperature structures of iron and cobalt elemental metals are not fcc. Both iron and cobalt undergo transformations and become fcc at high temperatures or in the presence of other elements alloyed with iron and cobalt. Nickel, on the other hand, is fcc at all temperatures. In superalloys based on iron and cobalt, the fcc forms of these elements thus are generally stabilized by alloy element additions, particularly nickel, to provide the best properties.

The upper limit of use for superalloys is not restricted by the occurrence of any allotropic phase transformation reactions but is a function of incipient melting temperatures of alloys and dissolution of strengtheningphases. Incipient melting is the melting that occurs in some part of the alloy that, when solidified, is not at equilibrium composition and thus melts at a lower temperature than that at which it might otherwise melt. All alloys have a melting range, so melting is not at a specific temperature even if there is no nonequilibrium segregation of alloy elements. Superalloys are strengthened not only by the basic nature of the fcc matrix and its chemistry but also by the presence of special strengthening phases, usually precipitates. Working (mechanical deformation, often cold) of a superalloy can also increase strength, but that strength may not endure at high temperatures.

Some tendency toward transformation of the fcc phase to stable lower-temperature phases occasionally occurs in cobalt-base superalloys. The austenitic fcc matrices of superalloys have extended solubility for some alloying additions, excellent ductility, and (iron-nickel- and nickel-base superalloys) favorable characteristics for precipitation of uniquely effective strengthening phases.

Pure iron has a density of 0.284 lb/in.3 (7.87 g/cm3), and pure nickel and cobalt have densities of about 0.322 lb/in.3 (8.9 g/cm3). Iron-nickel-base superalloys have densities of about 0.285 to 0.300 lb/in.3 (7.9 to 8.3 g/ cm3); cobalt-base superalloys, about 0.300 to 0.340 lb/in.3 (8.3 to 9.4 g/cm3); and nickel base alloys, about 0.282 to 0.322 lb/in.3 (7.8 to 8.9 g/cm3). Superalloy density is influenced by alloying additions: aluminum, titanium, and chromium reduce density, whereas tungsten, rhenium, and tantalum increase it. The corrosion resistance of superalloys depends primarily on the alloying elements added, particularly chromium and aluminum, and the environment experienced.

The melting temperatures of the pure elements are as follows: nickel, 2647 F (1453 C); cobalt, 2723 F (1495 C); and iron, 2798 F (1537 C). Incipient (lowest) melting temperatures and melting ranges of superalloys are functions of composition and prior processing. Generally, incipient melting temperatures are greater for cobalt-base than for nickel- or iron-nickel-base superalloys. Nickel-base superalloys may show incipient melting at temperatures as low as 2200 F (1204 C). Advanced nickel-base single-crystal superalloys having limited amounts of melting-point depressants tend to have incipient melting temperatures equal to or in excess of those of cobalt-base superalloys.

Invar 36 Application

For the manufacture of tooling for composites, Invar 36 offers a unique combination of matched thermal expansion to avoid warping during the curing cycle, along with strength, rigidity, durability, ease of fabrication and availability.

Inconel 625 Application

The properties of Inconel 625 that make it an excellent choice for sea-water applications are freedom from local attack (pitting and crevice corrosion), high corrosion-fatigue strength, high tensile strength, and resistance to chloride-ion stress-corrosion cracking. It is used as wire rope for mooring cables, propeller blades for motor patrol gunboats, submarine auxiliary propulsion motors, submarine quick-disconnect fittings, exhaust ducts for Navy utility boats, sheathing for undersea communication cables, submarine transducer controls, and steam-line bellows. Potential applications are springs, seals, bellows for submerged controls, electrical cable connectors, fasteners, flexure devices, and oceanographic instrument components.