Heat-resistant and heatproof alloys

Heatproof alloys and steel - materials working under high temperature within a set period of time in a combined-stress condition and that have a good corrosion resistance in gas media.

Heat-resistant alloys and steel - materials working in an unloaded or lightly-loaded condition under high temperatures (over 550 °C) and that have a good corrosion resistance in gas media.

These materials became highly-demanded at the end of the 30s of the last century when the need in products working under rather high temperatures had emerged. This is related to the development of jet aviation and gas turbine engines.

Heat-resistant and heatproof alloys include nickel, cobalt, titanium, iron, copper and aluminum. Nickel alloys are the most widely used ones. They are divided into casting, deformable and powder types. The most common heatproof alloys are nickel-based complex ones. They can work under 1050-1100 °C during hundreds and thousands of hours under high static and dynamic load.

As we are talking about heat-resistant and heatproof steel and alloys, we need to define high-temperature strength and heat resistance.

Terms and definitions

High-temperature strength - the ability of steels and alloys to resist mechanical strength under high temperatures within a certain period. The term “heat stability” is usually used for temperatures up to 600°С. High-temperature strength can be given a more precise definition.

High-temperature strength is also stress that causes the set deformation that does not lead to destruction and can be withstood by the structure metal under a certain temperature within a certain period of time. If the time and stress are considered, the property is called “long-term strength”; if the time, stress and deformation are considered, it is called “creep strength”.

Creep - continuous deformation under constant stress. Long-term strength - resistance of the material to destruction under long-term impact of temperature.

Heat-resistance - stability towards gas corrosion of metals and alloys under high temperatures.

Classification

Several classifications of alloys and steel working under high and very high temperatures.

The following classification of heatproof and heat-resistant steel and alloys is the most general ones:

• Heat stable steel - work in loaded condition under temperatures up to 600°С during a long time. It includes carbon, low-alloy and chromium ferrite steel.
• Heatproof steel and alloys - work in loaded condition under high temperatures during a certain time and are heat-resistant enough. Chromium-nickel or chromium-nickel-manganese based austenite steel with alloying agents and nickel and cobalt-based alloys.
• Heat-resistant (non-scaling) steel and alloys - work in unloaded or lightly-loaded condition under temperatures exceeding 550°С and have a surface stability towards chemical decay in gas media. They include martensite chromium-silicon steel, chromium and ferrite chromium-aluminium steel, so as chromium- and nickel-based alloys.

Also they are classified by production method:

• casting;
• deformable.

The main useful property of heatproof alloys and steel is the ability to bear mechanical stress under high temperatures. There are different patterns of loading heatproof materials: static stretching, bending or torsion loads, thermal loads due to temperature changes, dynamic fluctuating loads of different frequency and amplitude, dynamic impact of high-speed gas streams on the surface. These materials should withstand corresponding loads.

The corrosive stability of the material in gas media under high temperatures is the main useful property of heat-resistant steel and alloys.

At the same time, technological properties play an important role from the point of view of producing ready-made products. Deformable alloys should have processing plasticity for pressurizing (so as under 700-800 °С( and casting alloys should have satisfactory casting properties (fluidness, porosity).

Nickel-based heatproof steel and alloys

Currently, nickel-based alloys are very important heatproof materials for working under 700-1100°С.

CrNi77TiAlB alloy (EI437B and EI437BUVD)
Chemical composition under GOST 5632-72, TC 14-1-402-72, % (by weight):

• EI437B alloys - 19-22 Cr; 2.4-2.8 Ti; 0.6-1.0 Al; <= 4.0 Fe; <= 0.4 Mn; <= 0.6 Si; <= 0.07 С; <= 0.01 В; <= 0.02 Се; <= 0.007S; <= 0.015 P; the rest - nickel;
• EI437BY alloys - 19-22 Cr; 2.5-2.9 Ti; 0.6-1.0 Al; <= 0.4 Mn; <= 0.6 Si; <= 0.07 C;<= 0.01 B; <= 0.02Ce; <= 0.007 S; <= 0.015P; the rest - nickel.

Technological data:

• the alloy is produced in open arc or induction furnaces with vacuum arc remelting;
• deformation temperature - beginning - 1180 °С, end - at least 900 °С, air cooling after deformation;
• recommended modes of thermal processing: CrNi77TiAlB (EI437B) - heating to 1080 °С, soaking - 8 h, air cooling; ageing under 700 or 750 °С, soaking 16 h, air cooling; CrNi77TiAlB (EI437BU) - heating to 1080 °С, soaking - 8 h, air cooling; ageing under 700 or 750 °С, soaking 16 h, air cooling.

CrNi70WMoTiAl alloy (EI617)
Chemical composition under GOST 5632-72, % (by weight): 13-16 Cr; 2-4 Мо; 5-7 W; 0.1-0.5 V; 1.8-2.3 Ti; 1.7-2.3 Al; <= 5 Fe; <= 0.5Mn; <= 0.6 Si; <= 0.12 С; <= 0.02 В; <= 0.02 Се; <= 0.015 S; <= 0.015 P>; the rest - nickel.

Technological data:

• the alloy is produced in arc and induction electric furnaces with vacuum arc remelting;
• deformation temperature - beginning - 1160, end - exceeding 1000 °С, air cooling after deformation;
• recommended modes of thermal processing: heating to 1190±10 °С, soaking - 2 h, air cooling; heating to 1050 °С, soaking - 4 h, air cooling; geing under 900 °С, soaking 16 h, air cooling;
• heating to 1180 °С, soaking - 6 h, air cooling; heating to 1000 °С, cooling with the furnace to 900 °С, soaking 8 h, air cooling; ageing under 850 °С during 15 h, air cooling.

Nickel- and iron-based heat-resistant steel and alloys

Nickel- and iron-based heat-resistant alloys are mainly used in gas turbines, furnaces and different high-temperature facility with working temperature up to 1350 °С. High oxidation resistance of steel and alloys is provided by high chromium content. E.g., CrNi70Al nickel-based alloy has the highest chromium content (by weight) - 26-29 %.

CrNi70Al (EI652) alloy
Chemical composition under GOST 5632-72, % (by weight): 26-29 Cr; 2.8-3.5 Al; <= 0.З Mn; <= 0.8 Si; <= 0.12 С; <= 0.1 Ва; <= 0.03 Се; <= 0.012 S; <= 0.015 Р; the rest is nickel.

Technological data:

• the alloy is melted in arc and induction electric furnaces;
• deformation temperature - beginning - 1190, end - exceeding 900 °С, air cooling after deformation;
• recommended mode of thermal processing - heating up to 1100-1200 °С, soaking - 10 min, air cooling;
• thin-section alloys can be welded by any method;
• the alloy can be deeply drawn, limiting drawing ratio K = D / (d + s) = 2.17 where D - blank diameter; d - punch diameter; s - wall thickness (in mm).

CrNi78Ti (EI435) alloy
Chemical composition under GOST 5632-72, % (weight): 19-22 Cr; <= 6 Fе; 0.15-0.35 Ti; <= 0.7 Mn; <= 0.8 Si; <= 0.12 С; <= 0.012 S; <= 0.015 Р; the rest is nickel.

Technological data:

• the alloy is melted in arc and induction electric furnaces;
• deformation temperature - beginning - 1160, end - at least 950 °С, air cooling after deformation;
• recommended mode of thermal processing - heating up to 980-1020 °С, air or water cooling;
• the alloy can be welded by any methods;
• the alloy can be deeply drawn when die forging.

CrNi60WTi (EI868) alloy
Chemical composition under GOST 5632-72, % (by weight): 23.5-26.5 Cr; 13-16 W; <= 4 Fе; 0.3-0.7 Ti; <= 0.5 Mn; <= 0.8 Si; <= 0.5 Al; <= 0.1 С; <= 0.013 S; <= 0.013 Р; the rest is nickel.

Technological data:

• the alloy is melted in open arc or induction electric furnaces;
• deformation temperature - beginning - 1180, end - at least 1050 °С, air cooling after deformation;
• recommended mode of thermal processing - heating up to 1150-1200 °С, sheet soaking - 10 min, rod soaking - 2-2.5 h, air cooling;
• the alloy can be welded by any method;
• the alloy can be deeply drawn, limiting drawing ratio - 2.06.

CrNi65MoW (EP567), CrNi65MoWU (EP760)(hastelloy) alloys
Chemical composition under GOST 5632-72, % (by weight): 14.5-16.5 Cr; 15-17 Mo; 3-4.5 W; <= 1 Fе; <= 1 Mn; <= 0.15 Si; <= 0.03 С; <= 0.012 S; <= 0.015 Р; 59.8-67.5 Ni.

Semi-finished products made of the above-mentioned alloys undergo thermal processing - quenching under 1050-1090 °С and further water cooling.

They are applied for welding structures working under high temperatures in rather aggressive media (sulphuric acid, acetic acid, chlorides, etc.)

High-alloy steel

SV-06Cr15Ni60Mo15 (EP367) steel
Chemical composition under GOST 2246-70, % (by weight): 14-16 Cr; 14-16 Mo; <= 4 Fе; 1-2 Mn; <= 0.5 Si; <= 0.5 Al; <= 0.08 С; <= 0.015 S; <= 0.015 Р; the rest is nickel.

This steel does not belong to heatproof or heat-resistant ones, but are used for welding structures made of such alloys. They are applied for welding components made of nickel-based alloys (e.g., CrNi78Ti, CrNi70WMoAlTi, etc.) and welding heterogenous metals (e.g., chromium steel with nickel-based alloys). Build-up can be also applied besides welding.

Advantages:
• have good high-temperature strength;
• have good high resistance.

Drawbacks:
• chromium and nickel alloys are rather expensive;
• they include a lot of different components and their production is rather labor-intensive.

The above-mentioned materials are applied for producing rocket and space equipment, in gas turbines of aircraft, ship and power facility engines so as in petrochemical equipment. Such components include working blades, turbine discs, rings and other gas turbine elements so as combustion chambers, component nodes of furnaces and other products working under high temperatures for a long time. As a rule the range of working temperatures is 500-1350 °С. Semi-finished products made of some alloys are used as a filler for welding.

Heatproof and heat-resistant alloys are used for making different semi-finished products. Heatproof rods and round bars, wire and filaments, heatproof sheets and flat bars so as pipes. The above-mentioned semi-finished products are applied in different industries with high requirements towards high-temperature strength and heat resistance.