UNS N07751 Exhaust Valve Alloys Inconel® Alloy 751 Precipitation Hardenable

Alloy 751 (UNS N07751) for high power internal combustion engine exhaust valve

PRODUCT

High temperature Alloy 751 (UNS N07751) for high power internal combustion engine (diesel engine and gasoline engine) exhaust valves for automobile, locomotive, tractor, ship, tank, oil rig, construction machinery and mobile power station, etc.

EQUIVALENT DESIGNATION

Inconel® Alloy 751, VDM® Alloy 751 (Nicrofer 7016 TiAl), W.Nr.2.4694(DIN), NiCr16Fe7TiAl(DIN)

OVERVIEW

Alloy 751 is a precipitation hardenable nickel-chromium alloy, used mainly for the exhaust valves of internal combustion engines. The hardenability is obtained by means of addition of titanium, niobium and aluminum that form strengthening precipitates during heat treatment. It can be delivered in a solution annealed or hardened condition. It is generally used in the age hardened condition.

The alloy has essentially the same excellent physical, mechanical and corrosion resistance properties as alloy X-750.

In the evaluation of materials for exhaust valve applications, nickel-chromium alloys have performed well in a 1675°F (913°C) lead oxide screening test. Alloy 751 repeatedly offered good results, with corrosion rates averaging 4.31 g/dm²/h. Like other nickel-chromium alloys, alloy 751 also resists attack by other exhaust stream impurities such as sulfur, bromine and chlorine.

Alloy 751 is characterized by:

High tensile strength up to 600°C (1110°F)

High creep and fatigue strength up to 820°C (1510°F)

High oxidation resistance up to 980°C (1795°F)

Excellent mechanical properties in low temperature environments

Good corrosion resistance in high and low temperatures and high resistance against stress corrosion

Good weldability (resistance and fusion welding)

SKETCH OF EXHAUST VALVE

PRODUCTION PROCEDURE OF EXHAUST VALVE

Blanking → Electric heating upsetting forging of head blank → Heat treatment of head blank and rod → Friction welding → Rough turning or grinding → Finish turning → Cut fixed length → Semi-fine grinding the stem → The valve stem chrome plating → Fine grinding the stem → NDT of the finished valve → Delivery

SURFACE CONDITION OF EXHAUST VALVE

PRODUCTION SITE OF EXHAUST VALVE

ALL PRODUCT FORM OF ALLOY 751

Alloy 751 is available in forms of bar and rod, wire, strip, sheet, forging, circular blank, ring, extruded section, pipe and tube etc.

Rod and bar delivery condition: forged, rolled, drawn, heat treated, oxidized, descaled or pickled, twisted, peeled, ground or polished

Strip delivery condition: cold rolled, heat treated, pickled or bright annealed

Forged bar: 600mm diameter max.

Hot rolled bar: 120mm diameter max.

Cold drawn bar and rod: dia.4.0mm - 25.0mm

Bright bar and rod: dia.4.0mm - 25.0mm

Cold drawn wire: dia.0.10-10mm

Other shapes and dimensions, such as circular blanks, rings, forgings, extruded section, pipe and tube can be requested.

APPLICATION

Due to its high strength up to 820°C (1508°F) and its excellent corrosion resistance, Alloy 751 can be used in a wide range of applications. A typical application is exhaust valves of combustion engines.

CHEMICAL COMPOSITION (wt%):

PHYSICAL PROPERTY

Density:ρ=8.22 g/cm³

Melting range:1395-1430℃

MECHANICAL PROPERTIES

Typical mechanical properties of solution-annealed and precipitation hardened at room and elevated temperatures

CORROSION RESISTANCE

Alloy 751 demonstrates an excellent corrosion resistance in high and low temperatures and the alloy furthermore has a high resistance against stress corrosion. The resistance against oxidizing conditions is remarkably high at up to 980°C (1800°F).

MICROSTRUCTURE

Alloy 751 has a cubic, face-centered crystal structure. The excellent mechanical strength is due to the precipitation hardening in the gamma phase (γ) by formation of the gamma prime phase (γ’) along with some carbides during the hardening heat treatment.

FABRICATING

Alloy 751 can be easily deformed both hot and cold and can also be machined. However, for all processing, machines are required that take the mechanical properties like the high strength and characteristic work hardening rates into account. The deformation should preferably take place in the solution annealed condition.

Hot working

Sufficiently powerful equipment is important when hot-forming alloy 751 because of its resistance to deformation.

Alloy 751 can be optimally hot formed in a temperature range between 1200 and 980°C (2190 and 1800°F) with subsequent rapid cooling down. Cooling should be by water quenching or as fast as possible. For heating up, the workpieces are placed in the furnace that is heated up to the hot forming temperature.

Cold working

Alloy 751 is cold formed ideally in the solution annealed state. The material has a significantly higher work hardening rate than austenitic stainless steels. This must be taken into account for the design and selection of forming tools and equipment, and for the planning of forming processes.

Machining

While UNS N07080 in the solution-annealed condition is easier to process and the strain on tools is less, better surface quality is achieved in the age-hardened condition. The best results in terms of the surface quality and dimensional accuracy of the finished product are achieved by pre-treatment before hardening and by finishing in the age-hardened condition. For reasons of the increased tendency to work hardening in comparison with low-alloy austenitic stainless steels, a lower cutting speed should be selected and the cutting tool should stay engaged at all times. An adequate chip depth is important in order to cut below a previously formed work-hardened zone.

Nimonic 80A should be in the fully heat treated condition for all machining operations. The high material hardness in this condition (250-350 HV) requires the use of stringent machining techniques.

Heat treatment

In general, the heat treatment of Alloy 751 consists of two stages:

(1) Solution annealing between 980 and 1010°C (1796 and 1850°F) for min. 0.5 hours, followed by rapid cooling down.

(2) Hardening annealing between 700 - 780°C (1292 - 1436°F) for 2-4 hours followed by air cooling.

COMPETITIVE ADVANTAGE:

(1) More than 50 years experience of research and develop in high temperature alloy, corrosion resistance alloy, precision alloy, refractory alloy, rare metal and precious metal material and products.
(2) 6 state key laboratories and calibration center.
(3) Patent technologies.

(4) Average grain size 9 or finer.

BUSINESS TERM