High-alloyed and alloyed steels for high temperature applications:
Boiler, pressure vessel, and heat exchanger steels for power industry - characteristics and application
Also known as alloy and carbon steel for high temperature operation or as a steel for power industry, used in environments with operating temperatures up to 600 ℃, where the condition is to maintain similar strengths at room temperature. In metallurgy temperatures of 600 ℃ are referred to as elevated temperatures.
Unlike structural steel for treatment, spring steel or carburizing steel, boiler species have to cope with a rather responsible task - mainly to prevent or shorten the progression of deformation and fatigue of the exposed component during operation, to operating environment, load occurring during operation, and the frequency and time of load stress on the component.
Therefore, when selecting a particular grade from this subgroup, it is important to check that the material in this class will have sufficient resistance to the effects of hot gases, proper ductility, weldability, corrosion resistance and relaxation, and sufficient strength properties at operating temperature. The properties mentioned above, such as the yield point at a given operating temperature, the creep strength, the creep limit and, in addition, the heat resistance, are determined first.
Chemical composition and use of boiler steel
Boiler steels are used mainly in conventional or nuclear power plants and in the chemical industry, for example as superheated steam boilers, steam manifolds, steering discs, tubing, heat exchangers, screws, nuts and heavy loaded rivets, steam boilers, for components (eg blades) and fittings for water, gas and steam turbines.
One of the most important elements that makes boiler steels able to work at elevated temperatures and exhibits suitable strength properties is Chrom-Cr, Vanadium-V, and Molybdenum-Mo. With the increase of chromium in the chemical composition of the product, the corrosion resistance and the ability to use the component at higher operating temperatures increases. With a chromium content of 1.00-2.50%, they can be used continuously to a temperature of about 580 ℃. Chromium content above 2.5% up to 13% does not significantly improve creep resistance. In oxidizing atmospheres carbon steels exhibit resistance up to about 500 ℃.
As can be seen, the carbon content of various grades of boiler steel is comparable, and consequently strength properties such as yield strength and tensile strength do not significantly affect carbon, but other carbide elements such as vanadium, molybdenum and even additives Tungsten - N, Niobium - Nb, and Titanium - Ti, which simultaneously contribute to the increase of time creep strength.
Breakdown of boiler steel based on material structure
Steels for working at elevated temperatures are divided into two basic subgroups:
- ferritic boiler steels, that is:
- ferritic-pearlitic boiler steels,
- ferritic-bainitic boiler steels,
- ferritic-pearlitic-bainitic boiler steels,
- and bainitic boiler steels
- and martensitic steels:
- bainitic-martensitic steels,
- steels with high-tempered martensite structure
Considering the chemical composition of boiler steel we distinguish:
- low-carbon boiler steels with a carbon content of up to 0.25%, and
- medium-carbon boiler steels with a carbon content of over 0.25%
which we then divide into:
- low-alloy boiler steels for operation at elevated temperatures
- medium-alloy boiler steels for operation at elevated temperatures
- high-alloy boiler steels for operation at elevated temperatures
Heat treatment, delivery condition and structure of individual grades
With regard to the heat treatment of steels resistant to higher temperatures and the structure of the material after machining, the steels for boiler plates, boiler tubes and power rods are delivered to the power plants in the normal state after stress relief or in the normal state, which results in the felted ferritic structure is mixed and consequently only slight increase in strength (ferritic-perlitic-bainitic, ferritic-perlitic or ferritic-bainitic).
In turn, a large group of alloyed steels can be hardened to a martensitic structure and then relieved to the structure of sorbite, as is the case in alloy grades of steel for improvement. Therefore, depending on the chemical composition, you may encounter contradictions in the naming of certain grades - between boiler steels and steel for improvement (eg. 25HM). Heat-treated alloys are usually alloyed and high-alloyed, and in particular high-chromium alloys in the form of products with much larger cross-sections such as forgings, shafts or forged bars. Note, however, that in grades with low hardenability, the products of smaller sections should be improved to obtain suitable mechanical properties.
For forgings of shafts and rotors of turbines, and forgings of turbine blades, bainitic steels are used. In order to achieve this structure, steels which can not be quenched after austenitization are used. They are characterized by the highest durability parameters of this group.
Creep resistance, relaxation and fatigue of boiler steel
The strength properties of the boiler steels could be clearly determined at operating temperatures up to 200 ℃. However, the use and naming of high temperature steel strongly motivated the testing of strength at 500-600 operating temperatures. It is required for boiler steel to have the highest possible creep strength or creep resistance parameter - i.e. the longest constant load time for a component with the lowest degree of deformation. Creep tests and tests were carried out in accordance with PN-75/H-04330 and the requirements of PN-75/H-84024.
The fatigue of turbine rotors in power engineering is a process occurring at room temperature and at elevated temperatures. As a results of high temperature differences (i.e. frequent heating and cooling), or work only at high temperatures, changes in stresses, pressures, load pressures associated with the rotation of the elements, the steel is susceptible to deformation, relaxation and notch phenomenon. The basic factor for fatigue testing is the working load cycle and the working time of the element. Fatigue of steel and determination of names according to PN-64/H-04325.
The third factor that is quite important in determining the usefulness of steel in the power industry is the relaxation phenomenon, i.e. the decrease or loss of elastic stresses in the parts of the machines, resulting from the higher temperature operating over a longer period of time, resulting in the leakage of the boiler pipes; in bolts and nuts irregular "clearances" are present. This property is tested and used in steel flanges of steam pipe lines, steel components for turbine hull bolts, steel components for shrinkage and pressurized joints, and steel components for vessels and pressure equipment.
Boiler steel - work environment and application
The most common gaseous media in power systems are air, steam and flue gases. Heat resistance depends on the type and composition of the gas occurring during the operation of the subassembly. Air, exhaust gases, superheated steam and high temperatures, and the combination of these factors acts on the oxidation steel.
The resulting oxides on the surface of boiler steel for power industry build up along with the temperature rise. In the boiler steels, the oxide layer rapidly breaks down and, at the same time, makes the oxide layer more difficult to produce, resulting in the destruction of the material (eg. St36K, St44K, K18, K20 grades - max 400-500 ℃). An element to prevent a procedure of this phenomenon is Chromium - Cr, which alloy addition of more than 0.5% in the composition longer protects the component exposed to the further progress of the oxidation phenomenon of the steels for power generation equipment to a temperature of approx. 550 ℃ (eg. 10CrMo9-10 - max. 580 ℃). Chromium oxides quickly and accurately cover the surfaces of the bars and boiler plates, and the forgings of boiler steels is increasing as the operating temperature increases. After complete coverage, the chromium oxide surface is practically unmovable and resistant to further material destruction. Chromium also exhibits resistance to various sulfur compounds in the exhaust gas, and, along with its chemical composition, also increases its working temperature.
Pipes, sheets and bars made of boiler steel for operation at elevated temperatures
The above-described boiler steels, steels for pressure equipment and steels for working at elevated temperatures are defined by standards PN-75/H-84024, PN-75/H-84019, PN-75/H-84030, BN-65/0631-06 and european Norms PN-EN 10269, PN-EN 10302, PN-EN 10088-1, according to which are supplied:
- Seamless heat exchanger boiler tubes according to PN-98/H-74252, BN-67/0648-02, BN-66/0648-06, BN-68/0648-12, BN-68/0648-11, PN-EN 10216-2, PN-EN 2017-2, PN-EN 2016-1, PN-EN 10217-5, PN-EN 10216-4,
- Boiler and pressure vessel plates according to PN-81/H-92123, PN-93/H-92132, PN-64/H-92148, PN-58/H-92137, PN- PN-57/H-92205, PN-EN 10028-2
- Forgings and forged bars from boiler steel according to PN-92/H-94009, PN-98/H-93238, BN-69/0661-02, BN-67/0663-01, PN-EN 10222-2
- Cold rolled boiler steel strips
- Hot-rolled boiler and pressure vessel steel bars according to PN-80/H-93015, PN-98/H-93238, PN-EN 10269, PN-EN 10273, PN-EN 10028-2, PN-53/H-93239, EN 10028-4,
- Boiler steel bright bars according to PN-80/H-93016, EN 10277
- Boiler steel drawn wires according to EN 10263-4