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Our company has one of the larg- est product portfolios in this sector. Our top quality products and efficient service contribute to the long-term success of our customers. Our top priority is to establish and maintain positive long-term relationships with our business partners. In order to support you in the stockist and project business, with good quality, innova- tive materials and modern production tech- nologies, our experts are constantly working to keep up with all the latest trends.
As a company operating successfully on an international level, we unite many nation- alities and cultures under one banner. Our network collaborates closely in all aspects of procurement, sales, production and logistics. Since our foundation as a joint venture, we hold our position in the top league of compa- nies in this sector. Thanks to our impressive product portfolio, we can open up attractive growth prospects. High temperature strength has become one of the most important attributes for the application of boiler tubes.
At the same time, economic aspects play a deci- sive role. The 18Cr-8Ni steel grade is a conventional austenitic stainless steel. With requirements for higher steam temperatures to increase the efficiency of power plants, the creep resistance of this steel is not sufficient today. Through the addition of about 3 wt. The addition of nitrogen leads to a solid-solution strengthening of the material.
This increases the allowable tensile stresses. The allowable stresses under creep conditions are mainly increased by the precip- itation of a Cu-rich phase in the matrix.
During service this phase is precipitating as well as the niobium carbonitride phase. The high temperature oxidation resistance is improved by shot peening of the inner tube surface. By this, a high density of grain and twin boundar- ies is created close to the surface. The chro- mium diffusion to the surface is accelerated and the formation of an adherent and dense Cr 2 O 3 layer is enhanced.
This protective layer reduces further oxidation to a minimum. DMV HCu beneficially utilises precipita- tion hardening to improve especially the creep properties of conventional 18Cr-8Ni austenitic stainless steels. Increasing the efficiency essentially results in a reduction of the fuel consumption and thus CO 2 emissions, which is a big challenge at the present time.
Efficiency is directly linked to the steam temperature and pressure. The materials must withstand these condi- tions throughout the whole service life of the component. This results in higher demands on creep resistance, elevated temperature strength and high temperature corrosion resistance. The thickness of the oxide layer inside the tubes may increase. Thick oxide scales are more susceptible to spalling, which can cause blockage in the super- heater tubes.
Furthermore, higher steam temperatures will lead to higher material temperatures, resulting in a higher creep strength demand. This aspect is specifically addressed and improved by the copper ad- dition in DMV HCu material compared to the conventional austenitic grade In future, steel development will continue to follow the new demands and challenges of boiler applications.
It is widely expected 5 1 Introduction that new power plants will run at even higher temperatures and pressures. All of these alloys contain higher nickel and chromium contents in order to meet the requirements for higher application temperatures and are thus more expensive. Finally, shot peening of the inner surface of the boiler tubes is performed, if requested. Differences in material properties and especially in the creep resistance compared to the conven- tional grade DMV are mainly achieved by addition of about 3 wt.
The formation of fine Cu-rich precipitates during service leads to an increased creep strength by the mechanism of precipitation hardening. Additionally, an increased carbon, niobium and nitrogen content are effective compared to the conventional grade. The material properties are influenced by the precipitation of niobium carbonitrides, M 23 C 6 carbides and NbCrN. A high phase stability during long- term aging is achieved . The production of boiler tubes is divided into three main processing steps.
A first thermal treatment is performed during the hot extru- sion of the material. This is followed by cold deformation to produce the final dimensions of the tube cold pilgering or cold drawing. A final heat treatment completes the produc- tion route. During this solution treatment, the precipitates are mainly dissolved in the austenitic matrix material. However, a certain amount of carbonitride precipitates remains, restricting the grain coarsening by pinning the grain boundaries.
Except for the small amount of niobium carbonitrides, the other elements and especially the copper are dissolved in the austenitic matrix. In the solution-annealed condition the dis- solved nitrogen increases the tensile strength at room temperature and elevated tempera- tures by the solid solution-strengthening effect.
Under creep conditions in service, precipitation of phases such as M 23 C 6 , niobium carbonitride and NbCrN starts to improve the creep resistance. However, the main effect on the creep rupture control is due to the coherent precipitation of a very fine Cu-rich phase in grains. This effect is most apparent after complete dissolution of copper during solution annealing. Thus, the full amount of copper can be utilised during exposure to creep conditions. DMV HCu exhibits a stable microstruc- ture after long-term exposure at elevated temperatures with only small amounts of brittle intermetallic phases such as sigma- 8 phase .
Nevertheless, a certain coarsening of the Cu-rich phases takes place during long-term service. If the strengthening effect is attributed to the coherency strain of the precipitates, the Cu-rich phases are less ef- fective if coarsening is too rapid. Thus, an op- timum copper content of 3 wt.
This shows typical creep damage in form of microcracks orientated perpendicular to the stress direction.
It is directed toward those areas that must be addressed to properly coordinate compositional specification, sampling practice, and test methods. Its use will help ensure that compositional requirements are clearly defined and that sampling practices and test methods are available to meet product specifications. Although it is recognized that individual laboratories are free to use other methods, the availability of ASTM approved methodology is essential for referee purposes and to demonstrate that properly equipped laboratories can make the required measurements. When existing sampling or test methods do not meet the needs of the new product specification standard, the specification-writing committee should request that the methods-writing committee develop the required standards. ASTM Committee E01 is responsible for test methods and practices covering the sampling and analysis of most metals, ores, and related materials. Scope 1.