CS253753B1 - Carburizing-resistant alloy - Google Patents

Abstract

The alloy contains carbon from 0.2 to 0.6 wt. %, nickel from 15 to 50 wt. %, phosphorus not more than 0.045 wt. %, sulfur not more than 0.05 wt. %, silicon from 0.5 to 2.5 wt. %, manganese from 0.05 to 0.5 wt. %, the rest iron and common impurities and its essence lies in the fact that it contains chromium in an amount from 18.1 to 35 wt. %. It also preferably contains aluminum in an amount from 0.3 to 1.5 wt. %. The alloy finds application in furnace systems of the chemical and petrochemical industries, especially in the conditions of pyrolysis furnaces.

Description

The invention relates to an alloy resistant to carburization, especially under pyrolysis furnace conditions.

In a carburizing environment such as pyrolysis gas, long-term exposure to high temperatures, i.e. up to 1,100 °C, leads to carbon saturation of steels and high-alloy alloys. This process is accompanied by an increase in the volume of the metal, which causes internal stress and, together with the degradation of other material properties and the existence of external forces in pyrolysis loops, causes damage to the pipes.

Currently, the service life of pyrolysis tubes is about 2 to 5 years and ends with the fracture of the brittle carbonized steel, or the tubes are discarded as a precaution after carburizing about 60% of the wall thickness. The decisive factor for the service life of the tubes is therefore the resistance to carburization in the pyrolysis environment.

In operation, in addition to carburization from pyrolysis gases, carbon also deposits on the surface of the pipes. This carbon is burned off at certain time intervals, e.g. by water vapor and air. During this oxidation period or even during operation, an oxide film forms on the surface, which prevents the diffusion of carbon into the steel. If this oxide film is stable, adherent and free of defects, then the steel has greater resistance to carburization.

Therefore, steels or alloys for pyrolysis tubes are alloyed with elements that create good protective layers. It is mainly silicon, aluminum and chromium that guarantee the formation of a stable oxide film. Elements that reduce the solubility of carbon in steel, such as an increased nickel content, also have a positive effect in this sense. To improve heat resistance, these steels are also alloyed with carbide-forming elements, such as niobium and tungsten.

Currently, centrifugally cast pipes made of chromium-nickel steels and alloys with a chromium content of 25% by weight and nickel of 20% by weight, or chromium of 25% by weight and nickel of 35% by weight, alloyed with niobium and with a carbon content of approximately 0.4% by weight and a silicon content of no more than 2% by weight, are used for these purposes.

Steels with a higher silicon content, or possibly alloyed with aluminum, are also known.

In addition to the above types of steel, there is also information on the production of steels with a different ratio of chromium and nickel, containing, for example, 30% by weight of chromium and 30% by weight of nickel, or even wrought steels containing 21% by weight of chromium, 32% by weight of nickel and alloyed with titanium and aluminum.

However, when choosing a chemical composition, it is necessary to consider, in addition to carburization resistance, heat resistance, structural stability, thermal fatigue resistance and mechanical properties. One of the important prerequisites is good weldability.

Given the difficulty of direct field testing, laboratory testing methodology is essential for evaluating materials against carburization. So far, most considerations about the importance of individual elements on carburization resistance have come from the results of backfill tests.

The carburizing atmosphere in the backfill does not correspond to the carburizing atmosphere during pyrolysis. The conditions created in sealed ampoules containing graphite, a small amount of Fe ₂ O ₃ , filled with hydrogen are substantially closer to the operational tests. In this case, an atmosphere is created consisting of hydrocarbons with a small content of CO and graphite, i.e. conditions very close to operational ones.

Carburization resistance is assessed by the distribution of carbon content in steel depending on the distance from the surface. For comparison of steels and alloys with different content of alloying additives and impurities, not only different maximum carbon content, but also different course of carbon content occur. To evaluate the influence of chemical composition of individual materials, it is optimal to use the size of the relative carburization area, characterized by the original carbon content and the course of carbon content after the carburization test, under constant conditions.

Low resistance to carburization is eliminated by an alloy, intended especially for the conditions of pyrolysis furnaces, containing carbon from 0.2 to 0.6 wt. %, nickel from 15 to 50 wt. %, traces of phosphorus up to a maximum of 0.045 wt. %, traces of sulfur up to a maximum of 0.05 wt. %, silicon from 0.5 to 2.5 wt. %, manganese from 0.05 to 0.5 wt. %, the rest iron and common impurities according to the invention, the essence of which lies in the fact that it contains chromium in an amount of from 18.1 to 35 wt. %. The alloy preferably contains aluminum in an amount of from 0.3 to 1.5 wt. %.

Compared to steels produced to date, the steel according to the invention has a 50% higher resistance to carburization while maintaining its heat resistance, structural stability and weldability.

The chemical composition of a number of steels and alloys that were monitored is given in Table 1. The melts according to the invention are particularly marked here. It was confirmed that nickel partially increases the resistance to carburization. However, the addition of silicon and aluminum is of fundamental importance.

The new finding according to the invention, which leads to an increase in resistance to carburization in pyrolysis furnaces, is the positive effect of decreasing manganese. Common manganese contents in cast steels for pyrolysis range from 1 to 1.5 wt. depending on the type of raw materials. By their selection or metallurgical process, the manganese content can be reduced to a maximum of 0.5 wt. % while maintaining the chromium content from 18.1 to 35 wt. % and thus significantly increase the resistance to carburization. Examples are steels and alloys according to Table No. 1 with different contents of silicon, manganese, chromium and nickel without or with the addition of niobium, aluminum and cerium. An example of the results of the course of the carbon content depending on the distance from the surface is shown in Fig. No. 1 and 2. In comparison with the chemical composition according to Table No. 1, the influence of some alloying elements can be evaluated. These data can be assessed more precisely in terms of the dependence of the carburized surface on the chemical composition of the melt, if we evaluate the differences in the content of silicon and manganese, or if we take into account the sum of silicon and aluminum reduced by the manganese content - Fig. No. 3. Steels, or alloys with a carbon content of 0.2 to 0.6% by weight, manganese 0.05 to 0.5%, nickel from 15 to 50% by weight, with a phosphorus content of max. 0.045% by weight, sulfur max. 0.05% by weight, silicon from 0.5 to 2.5% by weight, the rest being iron and common impurities, show improved resistance to carburization with a content of iron in the range from 18.1 to 35% by weight.

The alloy according to the invention has applications in furnace systems of the chemical and petrochemical industries.

Table 1

Chemical composition of the monitored series of melts

Chemical composition (wt. %) Graphic designation

Melting

carbon manganese silicon phosphorus sulfur chrome nickel niobium aluminum cerium Fig. 1, 2 fig 3 1 0.36 0.93 1.74 0.024 0.010 25.09 20.12 about ₂ */ 0.32 0.18 1.94 0.027 0.014 24.36 25.24 Δ 3 ^times/ 0.34 0.38 2.16 0.021 0.015 26.30 28.29 Λ 4 ^times/ 0.46 0.25 2.13 0.016 0.017 24.62 26.00 1.46 Δ 5 ,0.47 0.99 2.06 0.017 0.016 24.64 25.54 1.45 Λ 6 0.47 0.99 2.06 0.017 0.016 24.64 25.54 1.45 0.200 Δ 7 0.49 1.34 2.47 0.027 0.017 24.67 25.95 1.30 © Δ 8 0.53 0.56 2.35 0.030 0.019 25.25 25.87 1.29 0.007 Δ 9 0.58 1.46 2.40 0.018 0.031 25.35 26.25 1.65 0.62 Δ 10 ^times/ 0.46 0.25 2.13 0.016 0.017 24.62 26.00 1.46 Δ ll ^x/ 0.32 0.23 2.44 0.016 0.012 21.54 30.20 1.51 D 12 ^times/ 0.40 0.25 2.35 0.017 0.016 22.06 34.62 1.48 n α 13 0.50 0.94 2.02 0.014 0.008 21.11 36.01 1.41 0.026 □ 14 0.50 0.89 2.43 0.016 0.007 21.37 35.17 1.38 0.049 □ 15 ^times/ 0.37 0.49 1.04 0.019 0.010 20.91 35.41 1.08 16 0.36 0.64 1.13 0.014 0.011 19.88 35.39 1.50 1.13 0.330 17 0.42 0.95 2.35 0.015 0.015 20.85 35.60 1.49 1.32 18 0.49 1.06 2.48 0.016 0.009 21.14 35.31 1.39 1.25 © 0 19 0.49 0.93 2.05 0.014 0.009 21.30 35.01 1.44 1.19 The

Continuation of table 1

Chemical composition (wt. %) Graphic designation

Melting

carbon manganese silicon phosphorus sulfur chrome nickel niobium aluminum cerium Fig. 1, 2 fig 3 20 0.56 1.45 1.99 0.019 0.034 20.87 36.23 1.58 1.04 0 21 0.38 1.02 1.48 0.021 0.010 24.85 35.04 About 22 0.35 0.91 1.48 0.017 0.009 24.65 35.18 1.50 About 23 0.35 0.86 1.97 0.017 0.009 24.16 35.68 1.48 about

x /

Meltings according to the invention

Claims (2)

1. Carburetting alloy, in particular in pyrolysis furnace conditions, containing a carbon of from 0,2 to 0,6% by weight, a nickel from 15 to 50% by weight, a phosphorus trace of not more than 0,045% by mass of a trace sulfur of not more than 0,05 % by weight, silicon from 0.5% to 2.5% by weight, manganese from 0.05 to 0.5% by weight, the remainder iron and common impurities, characterized in that it contains chromium in an amount of 18.1 to 35 wt. 2. Alloy according to claim 1, characterized in that it preferably contains aluminum in an amount of from 0.3 to 1.5% by weight.