Monel K-500 Nickel Alloy Products High Hardness For Marine Service Virtually Non

Product Specification

Forged high strength corrosion resistant nickel copper alloy Monel K-500 round bar

MONEL® alloy K-500 (UNS N05500/ W.Nr. 2.4375) is a nickel-copper alloy which combines the excellent corrosion resistance of MONEL alloy 400 with the added advantages of greater strength and hardness. The increased properties are obtained by adding aluminum and titanium to the nickel-copper base, and by heating under controlled conditions so that submicroscopic particles of Ni3 (Ti, Al) are precipitated throughout the matrix. The thermal processing used to effect precipitation is commonly called age hardening or aging.

Typical applications for MONEL alloy K-500 products are chains and cables, fasteners and springs for marine service; pump and valve components for chemical processing; doctor blades and scrapers for pulp processing in paper production; oil well drill collars and instruments, pump shafts and impellers, non-magnetic housings, safety lifts and valves for oil and gas production; and sensors and other electronic components.

Chemical composition, %

Nickel (plus Cobalt) .....................................................63.0 min.

Carbon........................................................................0.25 max.

Manganese ...................................................................1.5 max.

Iron................................................................................2.0 max.

Sulfur ..........................................................................0.01 max.

Silicon ...........................................................................0.5 max.

Copper ......................................................................27.0 - 33.0

Aluminum ..................................................................2.30 - 3.15

Titanium ....................................................................0.35 - 0.85

A useful characteristic of the alloy is that it is virtually nonmagnetic, 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 may 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 nonmagnetic properties of the material will be 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.

Mechanical properties

The low-temperature properties of MONEL alloy K-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. Thus the alloy is suitable for many cryogenic applications.

Bolting

MONEL alloy K-500 is approved by the ASME Boiler and Pressure Vessel Code as an acceptable material for use as bolting. Allowable stresses for Section VIII, Division 1 usage up to 500°F are presented in ASME Code Case 1192, latest revision.

Corrosion resistance

The corrosion resistance of MONEL alloy K-500 is substantially 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 (3500 ppm) hydrogen sulfide solutions at acidic and basic pHs (ranging from 1.0 to 11.0), U-bend specimens of age-hardened sheet showed no cracking. 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.

Heating and pickling

Two types of annealing procedures are performed on MONEL alloy K-500: solution annealing and process annealing. The treatments are different in both their purpose and procedure.

Solution Annealing

MONEL alloy K-500 is hardened by the formation of submicroscopic 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 should be solution-annealed to dissolve any phases that may have formed in the alloy during previous processing. Solution annealing is normally performed by heating hot-finished products to 1800°F and cold-worked products to 1900°F. To avoid excessive grain growth, time at temperature should be kept to a minimum (normally, less than 30 minutes). Heating (ramp) and cooling times must be kept to a minimum to avoid precipitation of detrimental phases. Cooling after solution annealing is normally accomplished by quenching in water.

Process Annealing

During mechanical processing in production and subsequent forming of alloy K-500 products,

intermediate process annealing may be required to soften the product. Such anneals recrystallize the structure and are typically conducted at temperatures between 1400°-1600 °F.

While higher temperatures will anneal the product, intermediate process annealing temperatures are limited to avoid excessive grain growth. Time at temperature must be limited to avoid the formation of secondary phases which can compromise the hardness of the aged alloy K-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. The user is cautioned that exposure at temperature for times greater than 1.5 hours is not recommended. Excessive exposure can result in the formation of titanium carbide (TiC). This compound is stable at the aging temperatures used to harden alloy K-500 such that the titanium cannot participate in the hardening reaction, the formation of Ni3(Ti,Al). Thus, the strength and hardness can be compromised.

Obviously, it is best to avoid the formation of the titanium carbide phase. If, however, the phase is formed as a result of improper processing, solution annealing at 2050°F for 30 minutes is required to dissolve the particles. It should be noted that this heat treatment will result 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.

The Federal Standard for alloy K-500, QQ-N-286, addresses only solution annealing. In-process annealing is left to the discretion of the heat treater. The stated solution annealing temperature range in Revision F is 1600° to 1900°F. Thus, if an alloy K-500 component must be solution annealed at 2050°F because of the presence of titanium carbide, it must subsequently be reduced in section thickness before final heat treatment (solution annealing + age hardening) to comply with the requirements of the specification. Revision G has amended the solution annealing requirement to a minimum annealing temperature of 1600°F. Thus, material solution-annealed at 2050°F 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 obtain 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 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 Rockwell B).

Hold for 16 hr at 1100° to 1125°F followed by furnace cooling at a rate of 15° to 25°F per hr to 900°F. Cooling from 900°F to room temperature 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 quenched or annealed forgings, for annealed or hot-rolled rods and large cold drawn rods (over 1½ in. diameter) and for soft-temper wire and strip.

2. Moderately cold-worked material (175-250 Brinell, 8-25 Rockwell C).

Hold for 8 hr 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 hr at temperature, particularly if the material has been coldworked only slightly. As a general rule, material with an initial hardness of 175-200 Brinell should be held the full 16 hr. Material close to the top figure of 250 Brinell (25 Rockwell C) should attain full hardness in 8 hr. These procedures are applicable to cold-drawn rods, half-hard strip, cold-upset pieces and intermediate-temper wire.

3. Fully cold-worked material (260-325 Brinell, 25-35 Rockwell C).

Hold for 6 hr or longer at 980° to 1000°F followed by cooling to 900°F at a rate not exceeding 15° to 25°F per hr. In some instances slightly higher hardnesses may be obtained (particularly with material near the lower end of the hardness range) by holding 8 to 10 hr 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 hr at each step. For example, procedure 1 could be 16 hr at 1100°F + 4 to 6 hr at 1000°F + 4 to 6 hr 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.

Pickling

Pickling is a standard method for producing a clean surface on alloy K-500.

Hot Forming.

Proper temperature during deformation is the most important factor in achievement of hot malleability. Maximum recommended heating temperature for hot working MONEL alloy K-500 is 2100°F. Metal should 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 should be subjected to prolonged soaking, the temperature should be reduced to or held at 1900°F until shortly before ready to work, then brought to 2100°F. When the piece is uniformly heated, it should be withdrawn. In the event of long delay, the work should be removed from the furnace and water-quenched.

The hot-working temperature range is 1600° to 2100°F. Heavy work is best done between 1900° and 2100°F; working below 1600°F is not recommended. To produce finer grain in forgings, the final reheat temperature should be 2000°F and at least 30% reduction of area should be taken in the last forging operation.

When hot working has been completed, or when it is necessary for MONEL alloy K-500 to cool before further hot working, it should not be allowed to cool in air but should be quenched from a temperature of 1450°F or higher. If the piece is allowed to cool slowly it will self-heat-treat (age-harden) to some extent, and stress will be set up that may 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 will be oxidized to a lesser degree and will be easier to pickle if it is quenched in water containing about 2% by volume of alcohol. Cold Forming. In the annealed condition, alloy K-500 can be cold-worked by standard procedures. Although the alloy requires considerable power to form, it has excellent ductility.

Machining

Heavy machining of alloy K-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. 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 temperatures and slow cooling rates involved.

Available products and specifications MONEL alloy K-500 is designated as UNS N05500 and Werkstoff Nr. 2.4375. It is listed in NACE MR-01-75 for oil and gas service. Alloy K-500 is furnished in a wide range of standard mill forms including pipe, tube, round bar, flat bar, forging. Popular forms and sizes are available from stock; many specialty products may be obtained from converters.

Available products and specifications

Bar, Rod, and Forgings - BS3075NA18 (Wire), BS3076NA18 (Rod and Bar), ASTM B 865 (Rod and Bar), DIN 17752 (Rod and Bar), DIN 17753 (Wire), DIN 17754 (Forgings), QQ-N-286 (Rod, Bar, and Forgings), SAE AMS 4676 (Rod and Bar), ASME Code Case 1192 (Rod and Bar), ISO 9723 (Bar), ISO 9724 (Wire), ISO 9725 (Forgings)

Pipe and Tube - BS3074NA18 (Seamless Pipe and Tube), DIN 17751 (Pipe and Tube)

Product size range
Diameter: 100mm - 600mm
Length: on request
 
Production Process