Low carbon steel is a widely employed material for manufacturing boiler pressure components, encompassing low carbon steel, low alloy structural steel, low alloy hot steel, austenitic stainless steel, and martensitic steel. These steels can be categorized into sheets and pipes based on their profiles. Table 15-1a provides information on the chemical composition, mechanical properties, and applicable ranges of commonly used boiler steels.
In Table 15-1a, low-carbon steel is further classified based on quality into ordinary carbon structural steel, high-quality carbon structural steel, and special boiler steel.
| Steel Type | Steel Grade | Standard | Chemical composition ，% | Tensile Strength σbMPa | Yield Strength σs MPa | elongation δ5 % | Impact Akv J | Strain aging impact toughness J | Application | ||||||||||
| C | Si | Mn | V | Nb(W) | Mo | Cr | Ni(Ti) | P | S | ||||||||||
| min | Working pressure MPa | Temperature（℃） | |||||||||||||||||
| Max | |||||||||||||||||||
| Low Carbon Steel Plate | Q235-A | GB700 | 0.14～0.22 | 0.30 | 0.30～0.65 | 0.045 | 0.050 | 375～460 | 185～235 | 21～26 | 27 | ≤1.0 | When the steam pressure exceeds 0.1MPa, it should not contact with the flame | ||||||
| Q235-B | GB700 | 0.12～0.20 | 0.30 | 0.30～0.70 | 0.045 | 0.045 | |||||||||||||
| Q235-C | GB700 | ≤0.18 | 0.30 | 0.35～0.80 | 0.040 | 0.040 | |||||||||||||
| Q235-D | GB700 | ≤0.17 | 0.30 | 0.35～0.80 | 0.035 | 0.035 | |||||||||||||
| 20g | GB713 | ≤0.20 | 0.15～0.30 | 0.50～0.90 | 0.035 | 0.035 | 380～530 | 185～245 | 22～26 | 27 | 29 | ≤5.9 | ≤450 | ||||||
| 20R | GB6654 | ≤0.22 | 0.15～0.30 | 0.35～0.80 | 0.035 | 0.035 | 400～530 | 195～245 | 22～26 | 27 | ≤450 | ||||||||
| Low Carbon Steel Tube | 20 | GB3087 | 0.17～0.24 | 0.17～0.37 | 0.35～0.65 | ≤0.25 | ≤0.25 | 0.035 | 0.035 | 410 | 245 | 25 | ≤5.9 | Heating surface tube≤480Header pipe≤430 | |||||
| 20G | GB5310 | 0.17～0.24 | 0.17～0.37 | 0.35～0.65 | 0.030 | 0.030 | 410～550 | 245 | 35 | - | - | no limit | Heating surface tube≤480Header pipe≤430 | ||||||
| Low Alloy Steel Plate | 16Mng | GB713 | ≤0.20 | 0.20～0.55 | 1.20～1.60 | 0.035 | 0.030 | 440～655 | 245～345 | 18～21 | 27 | 29 | ≤5.9 | ≤400 | |||||
| 16MnR | GB713 | ≤0.20 | 0.20～0.60 | 1.20～1.60 | 0.035 | 0.035 | 450～655 | 265～345 | 18～21 | 27 | - | ≤5.9 | ≤400 | ||||||
| Low Alloy Heat Resistant Steel | 15CrMoG | GB5130 | 0.12～0.18 | 0.17～0.37 | 0.40～0.70 | 0.40～0.55 | 0.80～1.10 | 0.030 | 0.030 | 440～640 | 235 | 21 | 35 | no limit | ≤550 | ||||
| 12Cr1MoVG | GB5130 | 0.08～0.15 | 0.17～0.37 | 0.40～0.70 | 0.15～0.30 | 0.25～0.35 | 0.90～1.20 | 0.030 | 0.030 | 470～640 | 255 | 21 | 35 | no limit | ≤565 | ||||
| 12Cr2MoWVTiB | GB5130 | 0.08～0.15 | 0.45～0.75 | 0.45～0.65 | 0.28～0.42 | 0.30～0.55(W) | 0.50～0.65 | 1.60～2.10 | 0.08～0.18(Ti) | 0.030 | 0.030 | 540～735 | 345 | 18 | 35 | no limit | ≤600 | ||
| 12Cr3MoVSiTiB | GB5130 | 0.09～0.15 | 0.60～0.90 | 0.50～0.80 | 0.25～0.35 | 1.00～1.20 | 2.50～3.00 | 0.22～0.38(Ti) | 0.030 | 0.030 | 610～805 | 440 | 16 | 35 | no limit | ≤600 | |||
| Austenitic Stainless Steel Pipe | 1Cr18Ni9 | GB5130 | ≤0.15 | ≤1.00 | ≤2.00 | 17.00～19.00 | 8.00～10.00 | 0.035 | 0.030 | ≥520 | 205 | 35 | no limit | Endurance strength at 650 =63MPa | |||||
| 1Cr19Ni11Nb | GB5130 | 0.04～0.10 | ≤1.00 | ≤2.00 | Nb+Ti≥0.80～1.00 | 17.00～20.00 | 9.00～13.00 | 0.030 | 0.030 | no limit | Endurance strength at 650 =63MPa | ||||||||
Q235A~D refers to ordinary carbon structural steel, with the nomenclature derived from yield strength values, quality grade symbols, and deoxidation methods. The letter 'Q' represents the first letter of the Chinese phonetic alphabet for yield strength, followed by the actual yield strength value. Quality grades range from A to D, with the steel quality progressively increasing from A to D.
20 steel is classified as high-quality carbon structural steel, characterized by an average carbon content denoted by the number 20. The 'S' and 'P' indicate the presence of non-metallic inclusions in high-quality carbon structural steels with a carbon content of 0.20% (20%). This steel exhibits excellent plasticity, toughness, and processability. Special boiler steel, derived from carbon structural steel, shares similarities with high-quality carbon steel structure. In the naming convention, the carbon content follows the lowercase Chinese alphabet letter 'g'—for instance, 20g refers to No. 20 special boiler plate. For pipes, the carbon content follows the Chinese alphabet letter 'G,' and 20G denotes 20 dedicated boiler steel pipe.
While the chemical composition of this type of steel is similar to that of high-quality carbon structural steel, the mechanical performance requirements are more stringent. Specifically, impact toughness is subject to rigorous demands, necessitating low sensitivity. During smelting, it is crucial to achieve effective deoxidation and control non-metallic inclusions. The organizational structure should be uniform, with the grain size ideally controlled between 3 to 7 to minimize defects like sandwiching, shrinkage, looseness, and pores. Additionally, 20R is designated as 20 container steel, exhibiting chemical composition and mechanical properties close to 20g. When used to manufacture boiler pressure components, it is advisable to conduct a strain aging impact test.
low-alloy structural steel
Low carbon steel exhibits excellent plasticity, toughness, and processing capabilities, but it possesses lower strength and limited corrosion resistance. By introducing a small amount of alloying elements, totaling less than 3%, to conventional low carbon steel, one can achieve heightened strength, improved toughness, excellent weldability, and enhanced corrosion resistance, transforming it into ordinary low-alloy structural steel. Manganese serves as the primary element in this type, typically ranging from 1% to 1.6%. Manganese not only induces a robust solid solution strengthening effect but also refines ferrite grains, reduces pearlite thickness, and eliminates coarse flake carbides along the grain boundaries, thereby boosting the steel's strength and toughness.
In addition to the main alloying elements, small quantities of vanadium, niobium, titanium, and other auxiliary elements are typically added. These additional elements form fine carbides or nitrides in the steel, preventing the growth of austenite grains during heating. This facilitates the formation of fine ferrite grains and contributes to the precipitation strengthening effect, further enhancing the strength and toughness of the steel.
| Steel Grade | Chemical Composition（%） | ||||||||
| C | Si | Mn | Ni | Cr | Mo | Nb | P | S | |
| 19Mn5 | 0.17～0.22 | 0.30～0.60 | 1.0～1.30 | ≤0.03 | ≤0.30 | ≤0.10 | ≤0.040 | ≤0.040 | |
| 19Mn6 | 0.15～0.22 | 0.30～0.60 | 1.0～1.60 | ≤0.03 | ≤0.25 | ≤0.10 | ≤0.035 | ≤0.030 | |
| BHW35 | ≤0.15 | 0.10～0.50 | 1.0～1.60 | 0.6～1.0 | 0.2～0.40 | 0.2～0.40 | 0.005～0.020 | ≤0.025 | ≤0.025 |
| A299 | ≤0.30 | 0.15～0.40 | 0.9～1.50 | ≤0.25 | ≤0.25 | ≤0.08 | ≤0.035 | ≤0.040 | |
In China, 16Mng steel plate is widely utilized in low-pressure boilers due to its commendable mechanical properties, welding performance, process efficiency, and low-temperature impact toughness. Its medium and low-temperature mechanical properties surpass those of low carbon steel, and it exhibits better corrosion resistance, although it is less gap-sensitive than low carbon steel. However, the presence of gaps may reduce fatigue strength and increase susceptibility to cracks.
Table 16MnR represents a low-alloy container steel plate, requiring a strain aging impact test during the manufacturing of boiler pressure components.
In large-scale boilers in China, widely used steels include 19Mn5, 19Mn6, BHW35, and A299, with their chemical compositions provided in Table 15-1b. 19Mn5 and 19Mn6 are German-produced carbon-manganese ordinary low-alloy steels, featuring stringent control over harmful elements such as sulfur and phosphorus (not exceeding 0.02% in mass fraction). Utilizing vacuum degassing technology during smelting significantly reduces gas content in the steel. These steels exhibit excellent overall mechanical properties, welding performance, and process characteristics, as depicted in Table 15-1c, with superior temperature strength compared to 16Mng, as shown in Table 15-1d.
| Steel Grade | σs（MPa）≥ | σb（MPa） | δ5（%） | ISO V notch impact power（0℃）J≥ | ||
| ＞60～≤100（mm） | ＞100～≤125（mm） | ＞125～≤150（mm） | ||||
| 19Mn6 | 315 | 295 | 295 | 490～630 | 20 | 31 |
| 19Mn5 | 289 | 275 | 261 | 490～630 | 20 | 49 |
| BHW35 | 390 | 380 | 375 | 570～740 | 18 | 31 |
| A299 | 276 | 515～655 | 16 | 27 | ||
BHW35 (13MnNiMo54), also manufactured in Germany, is a low-alloy high-strength steel with a well-balanced composition of multiple elements. The alloying elements are thoughtfully designed, contributing to a stable structure and enhanced strength through their interactive effects. This results in improved overall mechanical properties. The steel is commonly utilized in a normalized and high-temperature tempered state, characterized by a tempering bainite plus ferrite structure, often referred to as low bainitic steel (as indicated in Table 15-1c, Table 15-1e).
A299, part of the carbon-manganese-silicon series of U.S. steel, maintains strict control over harmful elements such as sulfur and phosphorus in its chemical composition. Actual sulfur and phosphorus content in the steel is kept below 0.015%, surpassing the standard value specified in Table 15-1b. The mass fraction of silicon and manganese also experiences minimal fluctuations, ensuring excellent weldability. Furthermore, the steel exhibits favorable overall mechanical properties, along with versatile performance in both hot and cold processing and notable strength at medium temperatures.
