| A number of factors regarding the use of titanium should be borne in mind during design and fabrication stages in order that best use is made of the properties of the material. These facts, which are considered in more detail in other parts of this Design Guide, are summarised here. Design It is essential that the special properties of titanium should be designed in from the initial stages. Titanium should not be merely substituted for another material in an already existing design. Corrosion Resistance Titanium relies for its corrosion resistance on the presence of a surface oxide film. Thus, the material is normally more suitable for use in oxidising environments or where oxygen is readily available than in reducing solutions. In general, titanium is only used where the corrosion rate is very low and it is therefore not necessary to provide a corrosion allowance when designing equipment. This enables relatively thin gauge sheet to be used for the lining of carbon steel vessels, heat exchanger end boxes and tube plates, pumps, valves, etc. It also allows for the use of thin wall tube in tubular heat exchangers and thin sheet in plate type heat exchangers. This reduces the cost of equipment and improves heat transfer. Titanium alloys have normally been developed to provide good strength and creep resistance but, generally, the corrosion resistance of these alloys is inferior to that of the commercially pure material. Alloys which have a better corrosion resistance include titanium/palladium, Ti-0.8%Ni-0.3%Mo (ASTM Grade 12) and certain members of the metastable beta alloys. Galvanic Coupling As with all metals, when titanium is in direct contact with a dissimilar metal in an electrolyte, a galvanic couple is set up and one or both of the metals may corrode more quickly than when not coupled together. In almost all cases, titanium is the more noble member of the couple and causes increased attack on the other metal, the extent of the attack depending upon the relative area ratios and the particular electrolyte in use. Generally, it is advisable to avoid situations where galvanic couples exist when designing equipment. Mechanical and Physical Properties Mechanical Properties The low modulus of titanium relative to its tensile properties necessitates making allowance for a greater amount of springback than in other materials in forming operations. Because of this low modulus, the cross sectional area required for a component in titanium must be somewhat greater than that of the same item in a ferrous material in order to achieve the same degree of stiffness. In addition, pipe supports and tube baffles in heat exchangers must be carefully designed to prevent excessive deflection. Thermal Properties The coefficient of thermal expansion of titanium is about 75% that of carbon steel and it is therefore advisable to bear this in mind when designing and fabricating equipment incorporating the two materials. The mechanical strength of the commercially pure grades of titanium falls off fairly rapidly at temperatures above 150-200°C. General Fabrication Galling Titanium has a greater tendency towards galling than stainless steel, both in contact with itself and with other metals. This necessitates suitable modifications to machining techniques and to the design of screw threads and bearing surfaces. Oxidation Since titanium is a reactive material and readily combines with oxygen from the atmosphere if heated to above about 600°C, it is normally not recommended for use above this temperature for prolonged periods. Hydrogen Attack The diffusion rate of hydrogen in titanium is very much more rapid than that of oxygen, and furnaces used for preheating the material before hot working operations are carried out should therefore have slightly oxidising atmospheres. This results in relatively thin oxide scales but the contamination in depth which would occur should hydrogen be present is avoided. Mechanical Forming Temperatures The softer grades of commercially pure titanium sheet in the annealed condition and beta alloys in the solution treated condition can be readily cold formed. For the harder commercially pure grades and the Ti2.5%Cu alloy (IMI 230) warm forming may be necessary while sheet in Ti-6%Al-4%V is best worked at 650-700°C or it can be superplastically formed at 900-950°C. Machining Titanium can be readily machined but account must be taken of its galling tendency and its relatively low thermal conductivity. The basic requirements are a rigid machine, a sharp cutting edge on the tool, and the use of slow, heavy cuts leaving room for swarf to escape. Copious lubricant flow is recommended. Titanium Clad Materials Plate is available which has a thin sheet of titanium explosively bonded onto a very much thicker plate of carbon steel. This is useful for high pressure, high temperature vessels and heat exchangers but does not provide an economic alternative to the use of either solid titanium or a loose titanium lining for less onerous duties. An alternative solution to the use of explosion clad plate is Resista-Clad material that has been developed over recent years. Welding Titanium can be welded by resistance spot and seam, TIG, MIG, plasma and electron beam welding techniques. The commercially pure grades, some of the low to medium strength alloys and metastable beta materials are considered fully weldable. While the higher strength alpha-beta alloys can be joined, the ductility in the welds is likely to be lower than that in the parent material. Titanium should not be welded to other metals by any of the normal fusion techniques because the intermetallic compounds so formed result in very brittle welds. Coating There is no practical method of electroplating or flame spraying titanium onto metal substrates to provide a corrosion resistant coating. Cost Although the cost per kilogram of titanium may be higher than that of many competitive materials, it is important to take account of the following facts: The density of titanium is only about half that of ferrous materials or of nickel and copper base alloys, thus the cost of the material on a volume basis is halved; No corrosion allowance is normally necessary with titanium, thus the amount of material required is reduced; Because of the corrosion resistance of titanium, unscheduled downtime of equipment is virtually eliminated and maintenance costs are much lower than with other materials; In most applications equipment made from titanium gives a long guaranteed service life. |