Decarburizing

Hydrogen at elevated temperatures can also attack the carbon in steel, forming methane bubbles that can link to form cracks. Alloying materials such as molybdenum and chromium combine with the carbon in steel to prevent decarburization by hydrogen (132). 

The development of a sharp COE texture in the finished strip requires complex control of numerous variables. The conventional commercial process (18) involves hot-rolling a cast ingot at ca 1370°C to a thickness of about 2 mm, annealing at 800—1000°C, and then cold-rolling to a final thickness of 0.27—0.35 mm in two steps of 70 and 50%, respectively, with a recrystallization (800—1000°C) aimeal in between. The cold-roUed strip is decarburized (800°C) to ca 0.003% C in mixtures of wet results in a primary recrystallized stmcture containing grains of the COE... 

Uses. The sinter oxide form is used as charge nickel in the manufacture of alloy steels and stainless steels (see Steel). The oxide furnishes oxygen to the melt for decarburization and slagging. In 1993, >100, 000 metric tons of nickel contained in sinter oxide was shipped to the world s steel industry. Nickel oxide sinter is charged as a granular material to an electric furnace with steel scrap and ferrochrome the mixture is melted and blown with air to remove carbon as CO2. The melt is slagged, pouted into a ladle, the composition is adjusted, and the melt is cast into appropriate shapes. A modification of the use of sinter oxide is its injection directiy into the molten metal (33). 

Two-coat—one-fire enameling processes have also reduced the need for heavy-metal etching and nickel flashing (nickel replacement) for direct-on enamel apphcation to decarburized steels. 

Stable oxides, such as those of clrromium, vanadium and titanium cannot be reduced to the metal by carbon and tire production of these metals, which have melting points above 2000 K, would lead to a refractoty solid containing carbon. The co-reduction of the oxides widr iron oxide leads to the formation of lower melting products, the feno-alloys, and tlris process is successfully used in industrial production. Since these metals form such stable oxides and carbides, tire process based on carbon reduction in a blast furnace would appear to be unsatisfactory, unless a product samrated with carbon is acceptable. This could not be decarburized by oxygen blowing without significairt re-oxidation of the refractory metal. 

To avoid decarburization and Assuring of the carbon and low-alloy steels, which is cumulative with time and, for all practical purposes irreversible, the limitations of the Nelson Curves should be followed religiously, as a minimum. Suitable low-alloy plate materials include ASTM-A204-A, B, and C and A387-A, B, C, D, and E, and similarly alloyed materials for pipe, tubes, and castings, depending upon stream temperatures and hydrogen partial pressures, as indicated by the Nelson Curves. 

Environments. Among the environmental factors that can shorten life under thermal fatigue conditions are surface decarburization, oxidation, and carburization. The last can be detrimental because it is likely to reduce both hot strength and ductility at the same time. The usual failure mechanism of heat-resistant alloy fixtures in carburizing furnaces is by thermal fatigue damage, evidenced by a prominent network of deep cracks. 

Decarburization results from hydrogen absorption from gas streams at elevated temperatures. In addition to hydrogen blistering, hydrogen can remove carbon from alloys. The particular mechanism depends to a large extent on the properties of other gases present. Removal of carbon causes the metal to lose strength and fail. 

In square-forged kellys, the decarburized zone has been removed from the corners of the fillet between the drive section and the upset to prevent fatigue cracks. Hexagonal kellys have machined surfaces and are generally free of decarburized zone in the drive section. 

Step 3. Methane gas evolves during decarburization corrosion and builds up pressure, causing hydrogen embrittlement metal fatigue and eventual tube ... 

Additional high temperature changes cause decarburization, wherein carbon in the ferrite phase of carbon steel can be oxidized to carbon dioxide. 

Various forms of high temperature corrosion including long-term overheating, decarburization, and hydrogen embrittlement... 

Overheating effects and high-temperature corrosion 240 decarburization 262... 

Chemical Resistance. TaC oxidizes rapidly in air at 800°C. Otherwise it is one of the most chemically stable carbides. It decarburizes when heated in hydrogen at very high temperatures (3000°C). It does not react with nitrogen up to 2700°C. It reacts at high temperature with Nb, Ta, and Mo. It is stable in nonoxidizing acids, but is attacked easily by HNO3 and HF and by melts of oxidizing salts. 

---