PROPERTIES OF STEELS,Steel Properties

The strength of mild steel can be improved by adding small amounts (not exceeding 0.1%) of niobium, which permits the manufacture of semikilled steels with yield points up to 280 N/mm2. By increasing the manganese content to about 1.5% the yield point can be increased up to 400 N/mm2. This provides better retention of strength at elevated temperatures and better toughness at low temperatures.

Corrosion Resistance
Equipment from mild steel usually is suitable for handling organic solvents, with the exception of those that are chlorinated, cold alkaline solutions (even when concentrated), sulfuric acid at concentrations greater than 88%, and nitric acid at concentrations greater than 65% at ambient temperatures [7].
Mild steels are rapidly corroded by mineral acids even when they are very dilute (pH less than 5). However, it is often more economical to use mild steel and include a considerable corrosion allowance on the thickness of the apparatus. Mild steel is not acceptable in situations in which metallic contamination of the product is not permissible.
Heat Resistance
The maximum temperature at which mild steel can be used is 550°C. Above this temperature the formation of iron oxides and rapid scaling makes the use of mild steels uneconomical. For equipment subjected to high loadings at elevated temperatures, it is not economical to use carbon steel in cases above 450°C because of its poor creep strength. (Creep strength is time-dependent, with strain occurring under stress.)
Low Temperatures
At temperatures below 10°C the mild steels may lose ductility, causing failure by brittle fracture at points of stress concentrations (especially at welds) [8,9]. The temperatures at which the transition occurs from ductile to brittle fraction depends not only on the steel composition, but also on thickness.
Stress relieving at 600-700°C for steels decreases operation at temperatures some 20°C lower. Unfortunately, suitable furnaces generally are not available, and local stress relieving of welds, etc., is often not successful because further stresses develop on cooling.
High-Carbon Steels
High-carbon steels containing more than 0.3% are difficult to weld, and nearly all production of this steel is as bar and forgings for such items as shafts, bolts, etc. These items can be fabricated without welding. These steels
Materials Selection Deskbook
are heat treated by quenching and tempering to obtain optimum properties up to 1000 N/mm2 tensile strength.
Low-Carbon, Low-Alloy Steels
Low-carbon, low-alloy steels are in widespread use for fabrication-welded and forged-pressure vessels. The carbon content of these steels is usually below 0.2%, and the alloying elements that do not exceed 12% are nickel, chromium, molybdenum, vanadium, boron and copper. The principal applications of these steels are given in Table 3.8.
Mechanical Properties
The maximum permissible loading of low-alloy steels according to the ASME code for pressure vessels is based on proof stress (or yield point), which is applicably superior to those of carbon steels. The cost of a pressure vessel in alloy steel may be more expensive than in carbon steel. However, consideration should be given to other cost savings resulting from thinner-walled vessels, which provide fabrication savings on weldings, stress relieving, transportation, erection and foundation. Table 3.9 compares mild-and low-alloy steels used for fabricating spherical gas storage tanks.
Corrosion Resistance
The corrosion resistance of low-alloy steels is not significantly better than that of mild steel for aqueous solutions of acids, salts, etc. The addition of 0.5% copper forms a rust-colored film preventing further steel deterioration; small amounts of chromium (1%) and nickel (0.5%) increase the rust
Table 3.8. Applications of Low-Carbon, Low-Alloy Steels [10]
0.5 Mo \ High creep strength for:
1.25 CrMo V 1. pressure vessels such as boilers operating
2.25 CrMo ( at elevated temperatures; and
6 to 12 CrMoVW/ 2. oil refinery vessels such as crackers and
reformers with high hydrogen pressures.
5 to 9% Cr for oil refinery applications involving
high-sulfur process streams, e.g., pipe stills.
CuCr (Corten) Rust-resisting steels for structural
applications.
2 to 9% Ni
for cryogenic applications.
Properties and Selection of Materials 65 Table 3.9. Comparison of Mild and Low-Alloy Quenched and Tempered Steels [11)
aStcel a was quenched and tempered to a tensile, strength of 830 N/mm2 and a yield strength of 670 N/mm2, and steel b to 620 and 450 N/mm.2, respectively. It was considered that the maximum thickness of metal that could be welded onsite was 38 mm.
resistance of copper steels still further. Low alloy steels have good resistance to corrosion by crude oils containing sulfur. This is illustrated by the data in Figure 3.3.
In operations involving hydrogen at partial pressures greater than 35 kgf/cm2 and temperatures greater than 250°C, carbon steels are decarborized and fissured internally by hydrogen [13]. Small additions of molybdenum prevent hydrogen attack at temperatures up to 350°C and pressures up to 56 kgf/cm2. For higher temperatures and pressures chromium/molybdenum steels (2.25 Cr, 0.5 Mo) are used. Figure 3.4 shows operating limits for steels in atmospheres containing hydrogen.