Microalloying

Perhaps the most important innovation of all is in the thermomechanical control processes, involving closely controlled simultaneous application of heat and deformation, to improve the mechanical properties, especially of ultra-microalloyed compositions. Processes such as controlled rolling are now standard procedures in steel mills. 

Gladman, T. (1997) The Physical Metallurgy of Microalloyed Steels (The Institute of Materials, London). 

R. Jayaram, M.K.Miller, An atom probe study of grain boundary and matrix chemistry in microalloyed NiAl, Acta metall.mater. 42 1561 (1994)... 

These materials are normally microalloyed with Cb, V, and/or Ti. Supplemental specifications agreed to by manufacturer and purchaser commonly establish chemistry more restrictive than the base specification, as well as plate rolling specifications and requirements for weldability (i.e., C-equivalent) and toughness. 

Additionally, microalloy versions of X42 and X52 grades appear to enhance resistance to hydrogen embrittlement. 

There are currently no standard API 5L microalloy grades. The purchaser would have to specify appropriate supplemental requirements to achieve the benefits of microalloy steel. 

The reformer tubes typically operate at maximum temperatures of 1,600°F to 1,700°F and are designed for a minimum stress-to-rupture life of 100,000 operating hours. A 35/25 Ni/Cr alloy is used that is modified with niobium and microalloyed with trace elements such as titanium and zirconium. Smaller tube diameters provide better heat transfer and cooler walls. This reduces tube and fuel costs and increases tube life. But more tubes increases the pressure drop. The optimum inside tube diameter is 4 to 5 in. The wall thickness may be as low as 0.25 inch with a length of 40 to 45 ft. The lane spacing between tube rows must be enough to avoid flame impingement from the burners. Typical spacing is 6 to 8 feet. 

I45. Parrini, C. DeVito, A. "High Strength Microalloyed Pipe Steels Resistant to P ydrogen-Induced Failures", Presented at Micon 78 Conf., Houston, Texas, April 3-5, 1978, Private Publ. by Italsider, Taranto, Italy, also, ASTM book STP 672, Baltimore,... 

HP microalloys were developed during the 1990s. The microalloys enhanced carburization resistance and improved high-temperature creep-rupture resistance.88... 

The microalloy tubes allow increased flux rates and higher reformer outlet temperatures. This in turn can make it possible to reduce the steam-to-carbon ratio while the hydrogen purity remains the same.86... 

Microalloying of Ni-base alloys with rare earths improves ductility, stress rupture life and creep resistance as shown in Table 12.9. The improved properties are due to desulphurisation caused by rare earths. Addition of cerium to inconel 600 and Hstalloy improves their hot workability. 

Chromium shows no ductility at room temperature and hence it is difficult to obtain desirable shapes with useful properties. But microalloying of Cr with Y, La, or other light rare earths results in improved yield strength and ductility. On the other hand addition of heavy rare earths such as Er, Dy, and Lu makes Cr more brittle. 

Description The feedstock (e.g. natural gas) is desulfurized, mixed with steam and converted into synthesis gas over nickel catalyst at approximately 42 bar and 800-850°C in the primary reformer. The Uhde steam reformer is a top-fired reformer with tubes made of centrifugal microalloy steel and a proprietary "cold outlet manifold," which enhances reliability. 

Limited data show that both welding practice and location of defects can dictate the hydrogen embrittlement snsceptibility of a weld. A study on microalloyed steel API 5L Grade X60 examined weld joints that were fabricated using either one or two weld passes." Fractnre mechanics specimens were extracted from the base metal, fusion zone, and heat-affected zone and tested in 7-MPa hydrogen gas. Results showed that Kj values measured in the weld fnsion zones were similar to valnes in the base metal, i.e., Rju was approximately 100 MPaVm in each region. In contrast, the heat-affected zones were more susceptible to hydrogen embrittlement, and Rju was difficult to measure. The heat-affected zone in the two-pass weld was most susceptible. 

Paralloy H39WM (25/35 Cr/Ni + Nb Microalloy) Paralloy Technical Paper. 

Stress corrosion cracking in heat treated weldments indicates that postweld heat treatments are sometimes ineffective. Sometimes this result can be attribnted to microalloying. (Microalloying is discnssed later.) In some cases, subsequent stress corrosion cracking can be traced to an improperly... 

Some users either prohibit the use of microalloyed carbon steels or place limits on both the carbon equivalent and the microalloying content. The microalloying limits vary from user to user, but usually fall in the range of 0.03 to 0.10 wt% for the sum of Ti, V, and Cb. ... 

This chapter summarizes important contributions on the AB5/AB and elemental hydrides. Examples of microalloying in LaNis- and FeTi-based intermetallic hydrides have been emphasized, and there has been some discussion on elemental hydrides. Classification of testing methods for durability of alloys/ intermetallics were presented, examples of intrinsic and extrinsic tests that include cycling and aging have been reported for AB5/AB and other alloys. More recent results on extrinsic tests on lightweight complex hydrides (imide-amide) that were cycled between Li2NH/LiNH2 phases were also reported. 

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