Monel K500

Monel nickel-copper alloy K-500 combines the excellent corrosion resistance characteristic of Mone 400 with the added advantages of greater strength and hardness. The increased properties are obtained by adding aluminumand titanium to thenickel-copper base, and by heating under controlled conditions so that submicroscopic particles of Ni3 (Ti, Al) are precipitated thoughout the matrix. The thermal processing used to effect precipitation is commonly called age hardening or aging.

Alloy K-500 has approximately twice the tensile strength and triple the yield strength of Alloy 400. The strength of Alloy K-500 is maintained to 1200 degrees Fahrenheit, but stays ductile and tough down to temperatureof -400 degrees Fahrenheit. Alloy K-500 also stays non-magnetic to -200 degrees Fahrenheit. Additional characteristics of Alloy K-500 include outstandingcorrosion resistance in a wide range of chemical and marine surroundings, from salts and alkalis, non-oxidizing acids to pure water. Alloy K-500 is non-magnetic and spark resistant. It is also recommended that Alloy K-500 be annealed when it is welded and that any weldments be stress relieved prior to aging.

Monel K500 is a precipitation-hardenable nickel-copper alloy that combines the excellent corrosion resistancecharacteristic of Monel 400 with the added advantage of greater strength and hardness. These amplified properties, strength and hardness, are obtained by adding aluminum and titanium to the nickel-copper base and by a thermal processing used to effect precipitation, typically called age hardening or aging.

This nickel alloy is spark resistant and non-magnetic to -200° F. However, it is possible to develop a magnetic layer on the surface of the material during processing. Aluminum and copper may be selectively oxidized during heating, leaving magnetic nickel rich film on outside. Pickling or bright dipping in acid can remove magnetic film and restore the non-magnetic properties

When in the age-hardened condition, Monel K-500 has a greater tendency toward stress-corrosion cracking in some environments than Monel 400. Alloy K-500 has approximately three times the yield strength and double the tensile strength when compared with alloy 400. Plus, it can be further strengthened by cold working prior to precipitation hardening. The strength of this nickel steel alloy is maintained to 1200° F but stays ductile and tough down to temperatures of 400° F. Its melting range is 2400-2460° F.

Monel K500 Chemical Composition

Monel K500 Corrosion Resistance

The corrosion resistance of Monel alloy K-500 is subtantially equivalent to that of alloy 400 except that, when in the age-hardened condition, alloy K-500 has a greater tendency toward stress-corrosion cracking in some environments. Monel alloy K-500 has been found to be resistant to a sour-gas environment. After 6 days of continuous immersion in saturated (3500ppm) hydrogen sulfide solutions at acidic and basic pH’s (ranging from 1.0 to 11.0), U-bend specimens of age-hardened sheet show no cracking. There was some tightly adherent black scale. Hardness of the specimens ranged from 28 to 40 Rc.

The combination of very low corrosion rates in high-velocity sea water and high strength make alloy K-500 particularly suitable for shafts of centrifugal pumps in marine service. In stagnant or slow-moving sea water, fouling may occur followed by pitting, but this pitting slows down after a fairly rapid initial attack.

Monel K500 Characteristics

• Corrosion resistance in an extensive range of marine and chemical environments. From pure water to non-oxidizing mineral acids, salts and alkalis.
• Excellent resistance to high velocity seawater
• Resistant to a sour-gas environment
• Excellent mechanical properties from sub-zero temperatures up to about 480C
• Non-magnetic alloy

Monel K500 Applications

• Sour-gas service applications
• Oil and gas production safety lifts and valves
• Oil-well tools and instruments like drill collars
• Oil well industry
• Doctor blades and scrapers
• Chains, cables, springs, valve trim, fasteners for marine service
• Pump shafts and impellers in marine service

Monel K-500 Fabrication

Monel K-500 is readily fabricated by standard commercial procedures.

Welding alloy K-500 is best accomplished by the gas-tungsten-arc welding process. It is recommended that Monel K-500 be annealed when it is welded and that any weldments be stress relieved prior to aging.

Heavy machining of this alloy 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. Therefore, the recommended practice is to machine slightly oversize, age-harden, then finish to size. During aging, a slight permanent contraction takes place, but little warpage occurs because of the low temperatures and slow cooling rates involved.

Monel K500 Specifications

UNS N05500
BS 3072-3076 (NA18)
ASME Boiler Code Section VIII
SAE AMS 4676
MIL-N-24549 DIN 17743, 17752, 17754
Werkstoff Nr. 2.4375
QQ-N-286
NACE MR-01-75

Monel K500 Mechanical Properties

Typical Room Temperature Tensile Properties of Annealed Material

Monel K500 Physical Properties

Monel K500 Heat Treatment

Annealing is performed both for softening of the matrix after working and for solutioning of the age-hardening phase. Adequate softening may be achieved with temperatures as low as 1400-1600°F, but heating at 1800°F for hot-finished products and 1900°F for cold-drawn products is recommended for optimum response to subsequent age hardening. Grain growth becomes fairly rapid above 1800°F, and if a fine-grained structure is desired heating time should be kept to a minimum at these higher temperatures.

For optimum aging response and maximum softness, it is important to obtain an effective water quench from the heating temperature without delay. A dely 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 about 2% by volume of alcohol to the water will minimize oxidation and facilitate pickling.

The following age-hardening procedures are recommended for achievement of maximum properties.

• 1.Soft material (140-180 Brinell, 75-90 RB). Hold for 16 hrs at 1100 to 1125°F followed by furnace cooling at the rate of 15 to 25°F per hr to 900°F. Cooling from 900°F to room tempertature may be carried out by furnace or air cooling, or by quenching, without regard for cooling rate. This procedure is suitable for as-forged and quenced or annealing forgings, for annealed or hot-rolled rods and large cold-drawn rods (over 1-1/2″ diameter) and for soft -temper wire and strip.
• 2. Moderately cold-worked material (175-250 Brinell, 8-25 RB). Hold for 8 hrs or longer at 1100 to 1125°F, followed by cooling to 900°F at a rate not to exceed 15 to 25°F per hr. Higher hardnesses can be obtained by holding for as long as 16 hrs at temperature, particularly if the material has been cold-worked only slightly. As a general rule, material with an initial hardness of 175-200 Brinell should be held the full 16 hrs. Material close to the top figure of 250 Brinell (25Rc) should attain full hardness in 8 hrs. These procedures are applicable to cold-drawn rods, haft-hard strip, cold-upset pieces and intermediate-temper wire.

• 3. Fully cold-worked material (260-325 Brinell, 25-35 Rc). Hold for 6 hrs or longer at 980 to 1000°F followed by cooling to 900°F at a rate not exceeding 15 to 25°F per hour. In some instances slightly higher hardnessmay be obtained (particularly with material near the lower end of the hardness range) by holding 8 to 10 hrs at temperature. This procedure is suitable for spring-temper strip, spring wire or heavily cold-worked pieces such as small, cold-formed balls.
  NOTE: Cooling may be done in steps of 100°F, holding the furnace 4 to 6 hrs at each step. For example, procedure 1 could be 16 hrs at 1100°F + 4 to 6hrs at 1000°F + 4 to 6 hrs at 900°F. Procedures described under 1,2, and 3, however, will usually give higher properties.
  In some instances it may be desired to decrease heat-treating time, either for cost saving or for obtaining intermediate properties. It is difficult to make specific recommendations which would cover the full range of possibilities. The best procedure is to make pilot tests on specimens which duplicate the cross section of the material to be hardened.

Material which has been heated for any appreciable length of time in the temperature range 1100°F to time and temperature of exposure. Overaged material will have lower mechanical properties than properly aged metal, and the properties cannot be raised by subsequent aging treatments. In order to strengthen overaged material, it must be solution-annealed (1800-1900°F) to redissolve the age-hardening constituents, and then re-aged. All benefits of cold work are lost in annealing. The highest strength obtainable is that corresponding to the annealed and aged condition.

Material that has been age-hardened to produce maximum hardness will 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 was carried out. There may be a small increase in properties if the rate of cooling in the original heat treatment was too rapid between 1050 and 800°F. If the hardened material is subsequently heated above 1100°F and then cooled, there will be a decrease in properties. Hardened Monel alloy K-500 has been subjected to long continued heating at 800°F. A further slow aging occured during the first month of exposure, but continued heating caused no further significant change in properties.

Machining
Heavy machining of alloy K-500 is best accomplished when the material is in the annealing condition or hot-worked andquenching condition. Age-hardened material, however, can be finish-machined to close tolerances and fine finished. The recommended practice, therefore, is to machine slightly oversize, age-harden, then finish to size. During aging, a slight permanent contraction (about 0.0002 in/in) takes place, but little warpage occurs because of the low temperature and slow cooling rates involved.