Chromium brittleness

Chromium Extremely oxidation-resistant lightest of refractory metals Lowest melting point of refractory metals, brittle at low temperatures... 

C) 370/656X brittleness after exposure to temperatures between about 700 to 1. OSO-F. stainless steels. chromium stainless steels, over 13% Cr and any 400 Series martensitic chromium stainless steels low in carbon content (high Cr/C ratio). complex chromium compound, possibly a chromium-phosphorus compound. chromium steels at temperatures above about 700 F (370 C) keep carbon up in martensitic chromium steels and limit Cr to 13% max. 

There is another interesting difference between the two irons. Ni-Hard (nominally 1 A Cr, 4 A Ni, 3C) has a matrix of the iron carbide that suiTounds the areas of the steel constituent. This brittle matrix provides a continuous path if a crack should start thus the alloy is vulnerable to impact and is weak in tension. In contrast, HC 250 (nominally 25 Cr, 2 AC) has the steel portion as the matrix that contains island crystals of chromium carbide. As the matrix is tougher. HC 250 has more resistance to impact and the tensile strength is about twice as high as that of Ni-Hard. jMoreover, by a suitable annealing treatment the... 

Type 315-This has a composition that provides a similar oxidation resistance to type 309 but has less liability to embrittlement due to sigma formation if used for long periods in the range of 425 to 815°C. (Sigma phase is the hard and brittle intermetallic compound FeCr formed in chromium rich alloys when used for long periods in the temperature range of 650 to 850°.)... 

Many metals are naturally brittle at room temperature, so must be machined when hot. However, particles of these metals, such as tungsten, chromium, molybdenum, etc., can be suspended in a ductile matrix. The resulting composite material is ductile, yet has the elevated-temperature properties of the brittle constituents. The actual process used to suspend the brittle particles is called liquid sintering and involves infiltration of the matrix material around the brittle particles. Fortunately, In the liquid sintering process, the brittle particles become rounded and therefore naturally more ductile. 

From Fig. 3.11, it can be seen that by increasing the chromium content while maintaining a limited amount of nickel-equivalent elements, first mixed martensite-ferrite structures are produced and then fully ferritic. This is 6-ferrite, that is a body-centred cubic structure stable at all temperatures. Relative to martensite it is soft, but it is also usually brittle. For this latter reason, usage has in the main been in small section form. This and some other disadvantages are offset for some purposes by attractive corrosion resistance or physical properties. 

The ferritic steel 430S17 has enhanced oxidation resistance and finds some applications in sheet form, but its strength at elevated temperature is low. The higher chromium (20-30%) ferritic types show excellent oxidation resistance, but have poor elevated-temperature strength and, being difficult to produce and fabricate, are not used in large quantity. Cast versions of 27-30% Cr are quite widely used, especially where oxidation resistance, coupled with abrasion resistance, is required when high carbon contents are utilised. Such alloys are brittle. 

Chromium is a silvery white/gray, hard, brittle noncorrosive metal that has chemical and physical properties similar to the two preceding elements in period 4 (V andTi). As one of the transition elements, its uses its M shell rather than its outer N shell for valence electrons when combining with other elements. Its melting point is 1,857°C, its boiling point is 2,672°C, and its density is 7.19 g/cm. ... 

Chromium is a hard, brittle metal that, with difficulty, can be forged, rolled, and drawn, unless it is in a very pure form, in which case the chromium is easier to work with. It is an excellent alloying metal with iron. Its bright, silvery property makes it an appropriate metal to provide a reflective, non-corrosive attractive finish for electroplating. 

This rather useful empirical expression is applicable to many electrodeposited materials [e.g., molybdenum, zinc, steel (10)]. The expression has been able, for instance, to provide an acceptable explanation for the phenomenon of the brittle cracking in chromium electrodeposits. It has been quite helpful in the general study and understanding of the functional connection between hardness and grain size values in many electrodeposits. 

Modification of the metal itself, by alloying for corrosion resistance, or substitution of a more corrosion-resistant metal, is often worth the increased capital cost. Titanium has excellent corrosion resistance, even when not alloyed, because of its tough natural oxide film, but it is presently rather expensive for routine use (e.g., in chemical process equipment), unless the increased capital cost is a secondary consideration. Iron is almost twice as dense as titanium, which may influence the choice of metal on structural grounds, but it can be alloyed with 11% or more chromium for corrosion resistance (stainless steels, Section 16.8) or, for resistance to acid attack, with an element such as silicon or molybdenum that will give a film of an acidic oxide (SiC>2 and M0O3, the anhydrides of silicic and molybdic acids) on the metal surface. Silicon, however, tends to make steel brittle. Nevertheless, the proprietary alloys Duriron (14.5% Si, 0.95% C) and Durichlor (14.5% Si, 3% Mo) are very serviceable for chemical engineering operations involving acids. Molybdenum also confers special acid and chloride resistant properties on type 316 stainless steel. Metals that rely on oxide films for corrosion resistance should, of course, be used only in Eh conditions under which passivity can be maintained. 

A large fraction of the iron and steel produced today is recycled scrap. Since scrap does not require reduction, it can be melted down directly in an electric arc furnace, in which the charge is heated through its own electrical resistance to arcs struck from graphite electrodes above it. The main problem with this process is the presence of tramps (i.e., copper from electrical wiring, chromium, nickel, and various other metals) that accompany scrap steel such as crushed automobile bodies and that lead to brittleness in the product. Tin in combination with sulfur is the most troublesome tramp. Only the highest quality recycled steel—specifically, steel with no more than 0.13% tramps—can be used for new automobile bodies, and usually reprocessed scrap has to be mixed with new steel to meet these requirements. 

Aluminium-niobium alloys are best produced by the Goldschmidt process. A product which contains about 8 per cent, of aluminium is harder than glass or quartz its density is 7 5.3 A brittle alloy of chromium and niobium is obtained by fusing green chromium oxide and niobium together in the electric furnace.1... 

The brittle pellets are placed in an open combustion tube and heated for one hour at 500°C in a circular oven. The apparatus described in (I) under chromium chloride is used when the ignition is complete, except that 25ml of bromine (unwarmed) is substituted for the carbon tetrachloride, and the exit end of the combustion tube is fitted with a wide-mouth receiver that carries an outlet tube for the carbon dioxide. The reaction is maintained at 600-700°C until all the bromine has been volatilized the dark olive-green scale of product partially sublimes into the cooler end of the tube and receiver in almost quantitative yield. 

Stainless steels are iron-based alloys which contain more than 12% chromium. A common composition contains 18% Cr and 8% Ni, and is designated as either 18 8 or type-304 stainless steel. Unlike ordinary carbon steel, the stainless steels in the 300 series do not become brittle at low temperatures. Stainless steel has a rather low thermal conductivity. It may be welded, brazed, or soldered and is machinable with some difficulty. Type 303 is the easiest to machine. 

Tin—Nickel, Alloy deposits having 65% tin have been commercially plated since about 1951 (135). The 65% tin alloy exhibits good resistance to chemical attack, staining, and atmospheric corrosion, especially when plated copper or bronze undercoats are used. This alloy has a low coefficient of friction. Deposits are solderable, hard (650—710 HV 5Q), act as etch resists, and find use in printed circuit boards, watch parts, and as a substitute for chromium in some applications. The rose-pink color of 65% tin is attractive. In marine exposure, tin—nickel is about equal to nickel—chromium deposits, but has been found to be superior in some industrial exposure sites. Chromium topcoats increase the protection further. Tin—nickel deposits are brittle and difficult to strip from steel. Temperature of deposits should be kept below 300°C. 

AP rifle bullets usually have a bullet tip filler (usually lead) which is designed to cushion the effect of the impact on the AP core, which is very hard and brittle and can break on impact without a cushioning effect. The AP core is also frequently surrounded with a thin sheath of lead between the core and the bullet jacket. The AP core is usually hardened steel such as tungsten/carbon, tungsten/chromium, manganese/molybdenum, chro-mium/vanadium, or chromium/molybdenum. 

Erosion Similar to abrasion cutting in ductile metal fracture (of brittle material) very small chips or particles (e.g., impellers, propellers, fans) Reduce fluid velocity to eliminate turbulence select harder alloy (high chromium) hard coatings such as cement lined pipe, rubber lining... 

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