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Monel Alloys

• satyendra
• February 28, 2023
• 2 Comments
• corrosion resistance, Inco, Monel alloy, Monel alloy 400, Monel alloy 405, Monel alloy 500, Nickel-copper alloy,

Monel Alloys

Monel are a group of alloys of nickel (from 52 % to 67 %) and copper, with small quantities of iron, manganese, carbon, and silicon. Monel alloys are also known as Monel metal. Monel alloys are not cupro-nickel alloys since they have less than 60 % copper. Monel was created in 1905 by Robert Crooks Stanley, who at that time worked at the International Nickel Company (Inco). Monel was named after company president Ambrose Monell, and patented in 1906. One ‘l’ was dropped, since family names were not allowed as trade-marks at that time. The trade-mark was registered in May 1921, and the name is now a trade-mark of Special Metals Corporation. Monel alloys are expensive alloys and their use is limited to those applications where they cannot be replaced with cheaper alternatives. Stronger than pure nickel, Monel alloys are resistant to corrosion by several aggressive agents which include rapidly flowing seawater. These alloys can be processed and fabricated readily by hot-working, cold-working, machining, and welding.

During the early years, Monel was born out of joint research into a more affordable route to nickel silver by three metallurgists, David H. Browne, Victor Hybinette, and Robert C. Stanley. It was ultimately Stanley who realized the lower oxidation point of the nickel-copper sulphide found in the Bessemerized matte, refining the first ingot where others had failed. In the process, he accidentally stumbled upon a highly corrosion-resistant super-alloy.

Monel alloy 400 exists in the industrial and built environment since 1905. It is a binary alloy of the same proportions of nickel and copper as is found naturally in the meteoritic nickel ore from the Sudbury (Ontario, Canada) mines and is hence considered a puritan alloy. It was one of the first nickel alloys, its history dating back to the original nickel-copper ore mined in Canada in the late nineteenth century. The composition of the original ore is roughly what the chemistry for Monel alloy 400 is today. This alloy is silvery in colour.

A short-lived architectural metal, Monel alloy 400 was replaced by its cheaper cousin, stainless steel, from the midn1930s. Monel alloy suffered from a lack of managerial support as focus shifted toward newer alloys. Extensive competition during the 1940s led th focus shifted towards newer alloys as extensive competition and nickel procurement issues harmed growth in the 1940s.This issue of nickel procurement led this alloy into relative obscurity by the 1950s. Today it remains a specialty alloy used mostly in the marine field.

Monel alloys are primarily nickel-copper alloys. There are several types of Monel alloys as given in Tab 1. However, the present popular grades are Monel alloy 400, Monel alloy 405, and Monel alloy 500. Monel alloy 400 is produced today by several companies worldwide as UNS N04400. Monel alloy 400 is also known as ‘Historic Monel’ and ‘alloy 400’. UNS designation for Monel alloy 405 is UNSD N04405 and that of Monel alloy 500 is UNS N05500.

Historical development of mining, smelting, and refining process of Monel alloy

From the very start of operations in Sudbury in Canada, ore was collected and separated into four grades namely (i) a mixed copper-nickel ore, (ii) copper pyrites, (iii) pyrrhotite or nickel ore, and (iv) diorite rock. Composition varied widely in the early days, with nickel ranging from 1.28 % to 8.12 % and copper ranging from 0.49 % to 15.71 % during the period between 1892 to 1899. It was at the Sudbury Creighton pit, mined from 1901, that veins of chalcopyrite and pentlandite were found in the pyrrhotite resulting in a 2.3:1 nickel copper ratio which became synonymous with Monel. The ore was hand sorted into three sizes, coarse, ragging, and fines, and was syphoned off from waste rock and initially roasted in yards for up to nine months to lower sulphur content from 30 % to around 7 %. Environmental air pollution forced a permanent replacement to roasting furnaces in 1929.

Roasted material was smelted alongside the ‘green ore’ and the ‘reverts’ using a 3.1:1 ratio in blast furnaces. As the top-grade surface ore was depleted during the war, sintering and reverberatory furnaces gradually gained importance in use from 1911. The product was finally Bessemerized in a converter to remove a 40 % iron content through slag which was dumped as waste until collection began in the 1970s. Shipped to the United States, early refining took place in the original location of Orford’s works at Bayonne, New Jersey. While no evidence remains of the exact method, the original patent from Ambrose Monell describes the calcining of the Bessemerized matte to remove the sulphur, reduction of the oxides in a reverberatory furnace with carbon, and then copper or iron to alter the recipe appropriately. Later, president John F. Thompson confirmed this, noting Stanley added magnesium, likely for increased ductility.

In 1909, Browne filed a patent using lime in an electric arc furnace to separate off sulphur through a calcium sulphide slag. At Huntington, the process was perfected. The matte was ground into coarse sand by a jaw-crusher, cone crusher, and ball mill before entering a calcining furnace for four hours to remove the remaining 20 % sulphur. The resultant oxide was mixed with charcoal and sent to open hearth reverberatory furnaces to cast pigs, before final purification and casting in an electric furnace to produce two-ton ingots.

Unlike steel, porosity, slag, folds, or cracks were removed by a cutting down process known as milling, before being passed to the hammer department for chipping off defects. The blooms were then reheated and placed through the rolling mill to produce billets needed at the rod or sheet mill for production.

The Huntington refining process was continually improved. A Russian, Otto Lellep, described a more efficient conversion process for refining nickel or nickel-copper matte through steadily reducing conditions at very high temperature as early as 1917. It comes as no surprise that the later methods of 1923 and 1926 were assigned to the International Nickel Company (Inco). The company subsequently filed a new patent in 1928 which blew superheated steam into the matte at much lower temperature, avoiding lining damage to the converter and removing Lellep from the equation. The use of the natural nickel-copper ore and matte continued until 1947, at which time the company shifted to a nickel sinter and copper ingot charge. Presently the process uses an air induction method for production. 

Monel alloy 400

Monel alloy 400 is primarily is a high temperature alloy with typical composition of 67 % nickel and 23 % copper. Small additions of manganese, silicon, and iron are made to improve the grade’s overall strength and corrosion resistance. The alloy is mainly known for its high corrosion resistance. The high strength of this alloy is over a wide temperature range. Monel alloy 400 is designated as EN 2.4360 – NiCu30Fe in EN standards, and ISO NiCu30 in ISO standards.

Monel alloy 400 is primarily made up of nickel and copper with small additions of manganese, silicon, and iron for improving the overall strength and corrosion resistance of the grade. The alloy is a solid solution of nickel and copper which can be hardened by cold processing and prevents attack in the variety of conditions. It is the only alloy which attains minimum corrosion in the all types of application environments. That is why, it is an ideal nickel- based superalloy. The limiting values of chemical composition are nickel (plus cobalt) – 63 % minimum, copper – 28 % to 34 %, carbon – 0.3 % maximum, manganese – 2 % maximum, iron – 2.5 % maximum, sulphur – 0.024 % maximum, and silicon – 0.5 % maximum.

Monel alloy 400 is a solid-solution binary alloy. As nickel and copper are mutually soluble in all proportions, it is a single-phase alloy. It has a face-centered cubic lattice structure with a lattice parameter of 3.534 A (Angstrom). Fig 1 shows the typical microstructure of this alloy. In the unetched condition, a polished sample shows only randomly dispersed non-metallic inclusions. These consist of metal sulphides or silicates. Under some conditions, graphite particles can also be present.

Fig 1 Microstructure of Monel alloy 400

Structurally, Monel alloy 400 is a single phase, solid solution strengthened material which can be tempered or cold worked to improve its mechanical properties. It offers superior mechanical features at the sub-zero temperature ranges. The firmness and hardness improve with minor impairment of ductility or impact strength. The alloy does not show ductile to brittle transformation when it freezes to the liquid hydrogen point.

The physical properties of Monel alloy 400 are density – 8.8 grams per cubic centimetre, melting point – 1,300 deg C to 1,350 deg C, coefficient of expansion for tension – 13.9 micrometres per metre deg C (at 20 deg C to 100 deg C), modulus of elasticity (i) for tension and for compression – 179,300 MPa, and (ii) for torsion – 65,500 MPa, Poisson’s ratio – 0.32, and Curie temperature – 21 deg C to 49 deg C. The modulus of elasticity in compression is the same as that in tension. It can be seen that the Curie temperature lies within the ambient range. It is affected by variations in chemical composition. The values given above represent the range which can be expected from normal production. Hence, some heats are magnetic at room temperature while others are not. If there is a strong requirement for non-magnetic characteristics, other alloys are to be considered. The effect of temperature on modulus of elasticity in tension is in Fig 2.

Fig 2 Effect of temperature on modulus of elasticity in tension

The mechanical properties of Monel alloy 400 depend on the type of product such as bar and rod, section, wire, sheet, plate, or pipe etc. and the processing which the material has undergone such as rolling, annealing, and stress relieving etc. For example, the mechanical properties of the alloy as rolled plate are tensile strength – 517 MPa to 655 MPa, yield strength (0.2 % offset) – 276 MPa to 517 MPa, elongation – 45 % to 30 %, hardness Brinell (3,000 kg) – 125 to 215, and hardness Rockwell B – 70 to 96, and the mechanical properties of hot rolled alloy as annealed plate are tensile strength – 482 MPa to 586 MPa, yield strength (0.2 % offset) – 193 MPa to 345 MPa, elongation – 50 % to 35 %, hardness Brinell (3,000 kg) – 110 to 140, and hardness Rockwell B – 60 to 76.  The high strength and toughness of Monel alloy 400 is maintained over a wide range of temperatures up to 400 deg C. Monel alloy 400 is normally supplied in the cold worked stress relieved condition below 60 mm diameter and in the hot worked stress relieved condition for larger sizes. This ensures the maximum mechanical strength, the optimum machinability, and the best surface condition for the ultimate application.

Monel alloy 400 is characterized by relatively low hardness ranged between 115 HV (Vickers hardness) and 250 HV depending on the finishing process, and this disqualifies the material from applying where it is going to be exposed to wear because of erosion, cavitation, or adhesive wear. One of the processes which lead to get wear resistant surface on metal substrate is boriding. In this process, boron atoms are diffused into the substrate to form metallic borides of high hardness. Depending on the substrate material, boriding is also carried out to increase the corrosion resistance.

Monel alloy 400 has excellent mechanical properties at sub-zero temperatures. There is an increase in strength and hardness at sub-zero temperatures with only a slight decrease in elongation or impact resistance, without a ductile to brittle transition temperature. Hence, Monel ally 400 is a good material for use at sub-zero temperatures since it does not transition from ductile to brittle even at the temperature of liquid hydrogen. This is in marked contrast to several ferrous materials which are brittle at low temperatures despite their increased strength. Hence, because of the absence of a ductile to brittle transition temperature the Monel alloy 400 is also suited for several applications, where ferrous metals cannot be used.

Monel alloy 400 has good shear strength and shows excellent ductility and tough fracture characteristics at temperature range of room temperature to -200 deg C as measured by tear test over this temperature range with the maximum load increasing considerably with decrease in temperature. As regards to impact strength, Monel alloy 400 is notable for its toughness, which is maintained over a considerable range of temperatures. Tension and torsion impact test samples show that complete fractures occurring in the tension impact test samples whereas the torsion samples remaining intact. Attempts to produce fractures in the torsion samples by reducing the minimum area by 75 % are not successful because of the toughness of the material. Impact tests on samples representing both longitudinal and transverse orientation in the plate, and on welded samples of hot-finished plate at liquid-hydrogen and liquid-helium temperatures show no evidence of brittle fractures with the welded samples getting fractured in the weld. Also, samples do not show any significant quantity of anisotropy.

As regards creep and rupture properties are concerned, Monel alloy 400 is useful at temperatures up to and including 540 deg C in the oxidizing atmospheres. Higher temperatures can be employed if the alloy is in a reducing environment.

Monel alloy 400 shows exceptional corrosion resistance to hydrofluoric acid and several reducing media. It is also normally more resistant to attack by oxidizing media than higher copper alloys. This alloy offers exceptional resistance to hydrofluoric acid in all concentrations up to the boiling point and hence, it is one of few alloys which can be used in contact with fluorine and hydrogen fluoride. This alloy is highly resistant to several forms of sulphuric and hydrochloric acids under reducing conditions, as well as to alkalis. It is perhaps the most resistant of all commonly used engineering alloys. It is also resistant against salts and organic acids. This versatility makes Monel alloy 400 suitable for service in a variety of environments. This alloy offers excellent resistance in flowing seawater while with stagnant conditions show inducement of crevice and pitting corrosion. This alloy is also resistant to stress corrosion cracking and pitting in the majority of fresh and industrial waters. These corrosion resistant properties make this alloy widely utilized in chemical and marine engineering.

Monel alloy 400 offers excellent corrosion resistance in a variety of environments. Corrosion rates are low, particularly in rapidly flowing brackish or seawater. The product also provides excellent resistance to stress-corrosion cracking in freshwater. It is also suited to several applications where ferrous metals cannot be used. Products made of this alloy, because of its chemical inertness, are characterized by a long lifetime. Also, because of its high corrosion resistance, Monel alloy 400 is used for constructions and functional parts where aggressive environment occur. These can be acid, alkali, and salt but also seawater and polluted urban air.  It is air contamination which force architects and engineers to use corrosion-resistant material for buildings covering. Monel alloy 400 is a perfect solution for this requirement. The first major roofing installation made of Monel alloy was Pennsylvania Railroad Station in New York City in 1909. The potential life of this roof was estimated as 300 years and lasted the life of the building.

Monel alloy 400 remains bright and free from discolouration when heated and cooled in a reducing atmosphere or quenched in an alcohol-water solution. Rate of cooling has no significant effect. The alloy forms an adherent oxide film if allowed to cool in air after heating. Both cold-worked and hot-worked Monel alloy 400 needs thermal treatment to develop the optimum combination of strength and ductility and to minimize distortion during subsequent machining.

The cold drawn stress relieved condition of Monel alloy 400 offers the best machinability and surface finish. Moreover, this material is easy to shape by forming and machining processes, contrary to other nickel super-alloys. Because of the alloys copper content, Monel alloy 400 is a viable, low-cost alternative when compared to commercially pure nickel.

Stress equalizing of cold-worked material causes an increase in the yield strength at 0 % offset without marked effects on other properties. Stress equalizing is done by holding for around 3 hours at a temperature of 300 deg C. Stress relieving reduces stresses without producing a recrystallized grain structure. This treatment is desired to get minimum distortion after metal removal. Heating for 1 hour to 2 hours at 540 deg C to 565 deg C relieves strains in either hot-worked or cold-worked products. Stress relief (540 deg C to 650 deg C for 1 hour, followed by slow cooling) is strongly desired as a precaution against stress-corrosion cracking in certain environments. Stress relieving slightly decreases tensile strength, yield strength, and hardness and slightly increases elongation.

Annealing can completely soften work-hardened material. Time and temperature needed depend on the quantity of previous cold work. In general, Monel alloy 400 is annealed by the open heating method by holding at 870 deg C to 980 deg C for 2 minutes to 10 minutes, whereas box annealing is done most satisfactory at 760 deg C to 815 deg C for 1 hour to 3 hours holding at temperature. Grain growth occurs when material is heated in the upper portion of the annealing temperature range.

Pickling can produce bright, clean surfaces on Monel alloy 400. The material remains bright and free from discolouration when heated and cooled in a reducing atmosphere or quenched in an alcohol-water solution. Rate of cooling has no significant effect. Monel alloy 400 forms an adherent oxide film if allowed to cool in air after heating. Both cold-worked and hot-worked monel alloy 400 needs thermal treatment to develop the optimum combination of strength and ductility and to minimize distortion during subsequent machining.

Monel alloy 400 is readily fabricated by standard processes. With respect to its resistance to hot deformation, Monel alloy 400 is softer than several steels. It can, hence, be hot-formed into almost any shape. The use of proper temperature during hot forming is important. The range of hot-forming temperatures is 650 deg C to 1,175 deg C. For heavy reductions, desired metal temperature is 950 deg C to 1,175 deg C. Light reductions can be taken down to 1,650 deg C. Working at the lower temperatures produces higher mechanical properties and smaller grain size. Prolonged soaking at hot-working temperatures is detrimental. If a delay occurs during processing, the furnace is required to be cut back to 1,000 deg C and not brought to temperature until operations are resumed. In no , the alloy is to be heated above 1,175 deg C since a permanent damage can result.

Heavy forging is not to be carried out very rapidly since it can cause the alloy to become overheated because of working. The use of an optical pyrometer is desired for the purpose of measurement. In hot-bending operations, the alloy is to be worked as soon as possible after removal from the furnace. Preheating tools and dies to around 250 deg C is helpful to prevent chilling the material while working. A controlled forging procedure is necessary to meet the requirements of some specifications for forged, hot-finished parts. Both the quantity of reduction and the finishing temperature are to be controlled in order to develop the desired properties.

Cold forming of the Monel alloy 400 is adaptable to virtually all methods of cold fabrication. The forces needed and the rate of work hardening are intermediate between those of mild steel and Type 304 stainless steel.

Monel alloy 400 can be machined at satisfactory rates with machine tools normally employed by the industry. In general, cold-drawn or cold-drawn and stress-relieved material is desired for best machinability and smoothest finish.

Monel alloy 400 can be readily joined and fabricated. By proper control of the quantity of hot or cold work and by the selection of appropriate thermal treatments, finished fabrications can be produced to a rather wide range of mechanical properties. The alloy offers good fabrication capabilities and can be hot or cold formed, readily machined and joined through welding, brazing, or soldering. It can be joined by a variety of processes including gas tungsten-arc, gas metal-arc and shielded metal-arc processes. In all of these processes thorough cleaning of the joint area is necessary to avoid embrittlement from such sources as lubricants and paints. The material is to be free of scale for best welding. Welding procedures are similar to those used for austenitic stainless steels. Neither preheating, nor post weld heat treatment are normally needed.

Joint design is similar to that used for austenitic stainless steels with two exceptions. The first is the need to accommodate the sluggish nature of the molten weld metal, necessitating a joint design sufficiently open enough to allow fuller filler wire access to fill the joint. Compared to carbon steels, nickel alloys and specialty stainless steels have a lower heat conductivity and higher heat expansion. These properties have to be taken into account by a larger root openings or root gaps (1 mm to 3 mm). Because of the viscosity of the welding material (compared to standard austenites) and the tendency to shrink, opening angles of 60-degree to 70-degree is to be provided for butt welds. The second is the high thermal conductivity and purity of the material which makes weld penetration lower than in austenitic stainless steel.

Monel alloy 400 is readily joined by conventional processes and procedures. Most of the conventional welding processes can be used to join Alloy 400 to itself or dissimilar alloys. The choice of welding product is dependent upon the materials being joined and the environment to which they are going to be exposed. Dissimilar joints of Monel alloy 400 and AISI 304 stainless steel are widely used in chemical, petrochemical, nuclear industries whose environments demand heat resistance, corrosion resistance, tolerance to thermal cycles and creep, and good mechanical properties. Apart from these requirements, the dissimilar welds result in saving of novel and expensive materials reducing cost thereby. Joining of Monel alloy 400 and AISI 316 stainless steel has wide applications in various engineering sectors.

Monel alloy 400 is widely used in several fields, especially in marine, chemical and hydrocarbon processing equipments. Typical applications are (i) valves and pumps, (ii) pump and propeller shafts, (iii) marine fixtures and fasteners, (iv) electrical and electronic components. (v) springs, (vi) gasoline and fresh water tanks, (vii) crude petroleum stills, process vessels and piping, (viii) heat exchangers, (ix) boiler feedwater heaters, and water feed and steam generator tubing in power plants, (x) deaerating heaters, (xi) splash zone sheathing, and (xii) offshore valves, pumps, fittings and fasteners.

Monel alloy 405

Monel alloy 405 (UNS N04405) is a nickel-copper alloy with high strength and toughness. Alloy 405 also has excellent corrosion resistance in a range of media including seawater, hydrofluoric acid, sulphuric acid, and alkalis. Because of its increased sulphur content, Monel alloy 405 has improved machinability. Monel alloy 405 is mainly used for automatic screw machine stock and is not normally recommended for other applications.

Monel alloy 500

Monel alloy 500 is a precipitation hardening nickel-copper alloy which retains the excellent corrosion resistance of Monel alloy 400 with the added advantage of increased strength and hardness which can be maintained up to 650 deg C. The higher mechanical properties arise because of the small additions of aluminium and titanium to the nickel-copper base, and by heating under controlled conditions so that sub-microscopic particles of Ni3 (Ti, Al) are precipitated throughout the matrix. The thermal processing used to effect precipitation is normally called age hardening or aging.

Typical Monel alloy 500 products are (i) chains and cables, fasteners, and springs for marine service, (ii) pump and valve components for chemical processing, (iii) doctor blades and scrapers for pulp processing in paper production, (iv) oil well drill collars and instruments, pump shafts and impellers, non-magnetic housings, safety lifts and valves for oil and gas production, and (v) sensors and other electronic components.

The composition of the alloy is nickel (plus cobalt) – 63 % minimum, copper – 27 % to 33 %, carbon – 0.25 % maximum, manganese – 1.5 % maximum, iron – 2 % maximum, sulphur – 0.01 % maximum, silicon – 0.5 % maximum, aluminum – 2.3 % to 3.15 %, and titanium – 0.35 % to 0.85 %.

Physical properties of Monel alloy 500 are (i) density 8.44 grams per cubic centimetres, (ii) melting range – 1,315 deg C, to 1,350 deg C, (iii) modulus of elasticity (tension) – 179,264 MPa, (iv) modulus of elasticity (torsion) – 65,500 MPa, and (v) Poisson’s ratio (aged material at room temperature) – 0.32. Fig 3 shows the effect of temperature on modulus of elasticity.

Fig 3 Effect of temperature on modulus of elasticity

A useful characteristic of the Monel alloy 500 is that it is virtually non-magnetic, even at quite low temperatures. It is possible, however, to develop a magnetic layer on the surface of the material during processing. Aluminum and copper can be selectively oxidized during heating, leaving a magnetic nickel-rich film on the outside of the piece. The effect is particularly noticeable on thin wire or strip where there is a high ratio of surface to weight. The magnetic film can be removed by pickling or bright dipping in acid, and the non-magnetic properties of the material is restored. The combination of low magnetic permeability, high strength, and good corrosion resistance has been used to advantage in a number of applications, notably oil-well surveying equipment and electronic components. Monel alloy 500 has been found to have exceptionally good dimensional stability, both in long-time exposure tests and in cyclic tests. This property of the alloy has led to its use in high-precision devices, such as gyros. Age hardening causes an initial volume contraction. An annealed rod contracts 0.00025 millimetres per millimetre (mm / mm) during aging.

The low-temperature properties of Monel alloy 500 are outstanding. Tensile and yield strengths increase with decrease in temperature while ductility and toughness are virtually unimpaired. No ductile-to-brittle transformation occurs even at temperatures as low as that of liquid hydrogen. Hence, the alloy is suitable for several cryogenic applications. Welds can be produced with the strength of age-hardened base metal with no serious loss in ductility if aging treatments are performed after welding annealed material. Welding of age-hardened material is to be avoided because of greatly reduced ductility.

Shear strength data of the Monel alloy-500 show that rivets can be made to develop exceptionally high strength by partial or full heat treatment prior to driving. The ratio of shear strength to ultimate tensile strength decreases very slightly with increasing hardness, indicating that the longer aging periods increase tensile strength more rapidly than shear strength. Aging for 4 hours at 582 deg C to 593 deg C followed by air cooling is desired for cold-headed rivets. This treatment is adequate to develop a shear strength of around 585 MPa in the shank. Monel alloy 500 is useful for corrosion-resistant springs at temperature up to 250 deg C.

Monel alloy 500 is an acceptable material for use as bolting in boilers and pressure vessels. It is produced by adding aluminum and titanium to the basic Monel nickel-copper composition. Suitable thermal treatments produce a sub-microscopic gamma prime precipitate throughout the matrix. Fig 4 shows typical microstructure of hot-rolled, as-rolled Monel ally 500.

Fig 4 Microstructure of Monel alloy 500

The corrosion resistance of Monel alloy 500 is substantially equivalent to that of Monel alloy 400 except that, when in the age-hardened condition, Monel alloy 500 has a higher tendency toward stress-corrosion cracking in some environments. Monel alloy 500 has been found to be resistant to a sour-gas environment. After 6 days of continuous immersion in saturated (3,500 ppm) hydrogen sulphide solutions at acidic and basic pHs (ranging from 1 to 11), U-bend samples of age-hardened sheet have shown no cracking. Hardness of the samples ranged from 28 RC (Rockwell C) to 40 RC. The combination of very low corrosion rates in high velocity sea water and high strength makes the Monel alloy 500 particularly suitable for shafts of centrifugal pumps in marine service. In stagnant or slow-moving sea water, fouling can occur followed by pitting, but this pitting slows down after a fairly rapid initial attack.

Two types of annealing procedures are performed on Monel alloy K500 namely (i) solution annealing, and (ii) process annealing. The treatments are different in both cases because of their purpose and procedure. In case of solution annealing, Monel alloy 500 is hardened by the formation of sub-microscopic particles of a secondary phase, Ni3(Ti,Al). Formation of the particles takes place as a solid-state reaction during an age-hardening (or precipitation-hardening) heat treatment. Prior to the aging treatment, the alloy component is to be solution-annealed to dissolve any phases which have formed in the alloy during previous processing. Solution annealing is normally performed by heating hot-finished products to 982 deg C and cold-worked products to 1,038 deg C. For avoiding excessive grain growth, time at temperature is to be kept to a minimum (normally, less than 30 minutes). Heating (ramp) and cooling times are to be kept to a minimum to avoid precipitation of detrimental phases. Cooling after solution annealing is normally accomplished by quenching in water. Fig 5 shows effect of water quenching from various annealing temperatures.

Fig 5 Effect of water quenching from various annealing temperatures

During mechanical processing in production and subsequent forming of Monel alloy 500 products, intermediate process annealing is sometimes needed to soften the product. Such anneals recrystallize the structure and are typically conducted at temperatures between 760 deg C to 870 deg C. While higher temperatures anneal the product, intermediate process annealing temperatures are limited to avoid excessive grain growth. Time at temperature is to be limited to avoid the formation of secondary phases which can compromise the hardness of the aged Monel alloy 500 product. Holding for one hour after the part has reached the set temperature and equalized is normally sufficient to soften the alloy product during processing. Exposure at temperature for times higher than 1.5 hours is not desired. Excessive exposure can result in the formation of titanium carbide (TiC). This compound is stable at the aging temperatures used to harden Monel alloy 500 such that the titanium cannot participate in the hardening reaction, the formation of Ni3(Ti,Al). Hence, the strength and hardness can be compromised. Obviously, it is better to avoid the formation of the titanium carbide phase. If, however, the phase is formed as a result of improper processing, solution annealing at 1,120 deg C for 30 minutes is needed to dissolve the particles. This heat treatment results in a large grain size which can somewhat compromise formability. However, the high-temperature solution treatment is necessary if the component is to develop full hardness and strength during the aging treatment.

If a Monel alloy 500 component is to be solution annealed at 1,120 deg C because of the presence of titanium carbide, it is to be subsequently reduced in section thickness before final heat treatment (solution annealing + age hardening) to comply with the requirements of the specification. Hence, material solution-annealed at 1,120 deg C can be aged without further reduction in section thickness and is acceptable if it meets the other requirements of the specification (mechanical properties, etc.) For optimum aging response and maximum softness, it is important to get an effective water quench from the heating temperature without delay. A delay in quenching or a slow quench can result in partial precipitation of the age hardening phase and subsequent impairment of the aging response. Addition of around 2 % by volume of alcohol to the water minimizes oxidation and facilitates pickling.

Material which has been heated for an appreciable length of time in the temperature range 590 deg C to 760 deg C gets overaged to an extent dependent on time and temperature of exposure. Overaged material has lower mechanical properties than properly aged metal, and the properties cannot be raised by subsequent aging treatments. In order to strengthen overaged material, it is to be solution-annealed (982 deg C to 1038 deg C) to redissolve the age-hardening constituents, and then re-aged. All benefits of cold work are lost in annealing. The highest strength achievable is that corresponding to the annealed and aged condition. Material which has been age-hardened to produce maximum hardness do not show an appreciable change in properties if again heated to or held at any temperature up to that at which the original heat treatment has been carried out. There can be a small increase in properties if the rate of cooling in the original heat treatment has been too rapid between 565 deg C and 425 deg C. If the hardened material is subsequently heated above 595 deg C and then cooled, there is a decrease in properties. Hardened Monel alloy 500 when subjected to long continued heating at 425 deg C, a further slow aging occurs during the first month of exposure, but continued heating causes no further significant change in properties.

Pickling is a standard method for producing a clean surface on Monel alloy 500. Proper temperature during deformation is the most important factor in achievement of hot malleability. Maximum desired heating temperature for hot working Monel alloy 500 is 1,150 deg C. The alloy is to be charged into a hot furnace and withdrawn when uniformly heated. Prolonged soaking at this temperature is harmful. If a delay occurs, such that the material is to be subjected to prolonged soaking, the temperature is to be reduced to or held at 1,035 deg C until shortly before ready to work, then brought to 1,150 deg C. When the workpiece is uniformly heated, it is to be withdrawn. In the event of long delay, the workpiece is to be removed from the furnace and water-quenched. The hot-working temperature range is 870 deg C to 1,150 deg C. Heavy work is best done between 1,035 deg C and 1,150 deg C and working below 870 deg C is not desired,

For finer grain in forgings, the final reheating temperature is to be 1,090 deg C and at least 30 % reduction of area is to be taken in the last forging operation. When hot working has been completed, or when it is necessary for Monel alloy 500 to cool before further hot working, it is not to be cooled in the air but it is to be quenched from a temperature of 790 deg C or higher. If the workpiece is allowed to cool slowly, it self-heat-treats (age-harden) to some extent, and stress gets set up which can lead to thermal splitting or tearing during subsequent reheating. In addition, quenched material has better response to age hardening, since more of the age-hardening constituent is retained in solution. The surface of the material is oxidized to a lesser degree and it is easier to pickle if it is quenched in water containing around 2 % by volume of alcohol. In the annealed condition, Monel alloy 500 can be cold-worked by standard procedures. Although the alloy needs considerable power to form, it has excellent ductility.

Heavy machining of Monel alloy 500 is best accomplished when the material is in the annealed condition or hot-worked and quenched condition. Age-hardened material, however, can be finish-machined to close tolerances and fine finishes. Hence, the desired practice is to machine slightly oversize, age-harden, then finish to size. During aging, a slight permanent contraction (around 0.0002 mm / mm) takes place, but little warpage occurs because of the low temperatures and slow cooling rates involved.

Monel alloy 500 products can be joined by conventional processes and procedures. Welding Monel alloy 500 is best accomplished by the gas-tungsten-arc welding (GTAW) process. Monel filler metal 60 is normally used. However, the weldments are not age hardenable and, hence, do not have strength matching that of the hardened base metal. Weldments needing strength similar to the aged base metal are to be deposited with filler metal of matching composition which is available from other suppliers.

Monel alloy 500 also offers an exceptionally high dimensional stability and, as with Monel alloy 400, its low temperature mechanical properties are very good with no transition temperature making the alloy suitable for several cryogenic applications. The corrosion resistance of Monel alloy 500 is essentially the same as that of Monel alloy 400, except when in the age hardened condition. Heat treated Monel alloy 500 has a higher tendency toward stress-corrosion cracking in certain environments. However, it has been found to be resistant in sour-gas environments. The combination of its high strength and excellent marine corrosion resistance in high-velocity seawater make Monel alloy 500 particularly suitable for marine shafts and centrifugal pumps. In stagnant or slow-moving seawater fouling can occur followed by pitting, but this pitting does slow down after a fairly rapid initial attack. Monel alloy 500 can be readily fabricated and can be hot and cold formed, although heavy deformation or machining is best achieved in the annealed condition. If heavy machining is to be carried out I,t is common practice to machine to near net shape and to precipitation harden prior to finish machining. This enables a better surface finish to be achieved as well as closer tolerances.