WELDING STEEL PLATE

(1) In plates up to 3/16 in. (4.8 mm) in thickness, joints are prepared with a space between the edges equal to the plate thickness. This allows the flame and welding rod to penetrate to the root of the joint. Proper allowance should be made for expansion and contraction in order to eliminate warping of the plates or cracking of the weld.

(2) The edges of heavy section steel plates (more than 3/16 in. (4.8 mm) thick) should be beveled to obtain full penetration of the weld metal and good fusion at the joint. Use the forehand method of welding.

(3) Plates 1/2 to 3/4 in. (12.7 to 19.1 mm) thick should be prepared for a U type joint in all cases. The root face is provided at the base of the joint to cushion the first bead or layer of weld metal. The backhand method is generally used in welding these plates.

NOTE

Welding of plates 1/2 to 3/4 in. (12.7 to 19.1 mm) thick is not recommended for oxyacetylene welding.

(4) The edges of plates 3/4 in. (19.1 mm) or thicker are usually prepared by using the double V or double U type joint when welding can be done from both sides of the plate. A single V or single U joint is used for all plate thicknesses when welding is done from one side of the plate.

GENERAL PRINCIPLE IN WELDING STEEL

(1) A well balanced neutral flame is used for welding most steels. To be sure that the flame is not oxidizing, it is sometimes used with a slight acetylene feather. A very slight excess of acetylene may be used for welding alloys with a high carbon, chromium, or nickel content. However, increased welding speeds are possible by using a slightly reducing flame. Avoid excessive gas pressure because it gives a harsh flame. This often results in cold shuts or laps, and makes molten metal control difficult.

(2) The tip size and volume of flame used should be sufficient to reduce the metal to a fully molten state and to produce complete joint penetration. Care should be taken to avoid the formation of molten metal drip heads from the bottom of the joint. The flame should bring the joint edges to the fusion point ahead of the puddle as the weld progresses.

(3) The pool of the molten metal should progress evenly down the seam as the weld is being made.

(4) The inner cone tip of the flame should not be permitted to come in contact with the welding rod, molten puddle, or base metal. The flame should be manipulated so that the molten metal is protected from the atmosphere by the envelope or outer flame.

(5) The end of the welding rod should be melted by placing it in the puddle under the protection of the enveloping flame. The rod should not be melted above the puddle and allowed to drip into it.

See Why Metals Corrode?

When discussing the ionic content of an aqueous medium, the question often arises as to how acid (or alkaline) is the solution. Quite simply, this refers to whether there is an excess of H⁺ (hydrogen) or OH^- (hydroxyl) ions present. The H⁺ ion is acid while the hydroxyl ion is alkaline or basic. The other ionic portion of an acid or alkali added to water can increases its conductivity or change other properties of the liquid, but does not increase or decrease its acidity. For instance, whether a given amount of H⁺ ion is produced in water by introducing hydrochloric (HCl), sulfuric (H₂SO₄), or any other acid is immaterial. The pH of the solution will be the same for the same number of dissolved hydrogen atoms. (reference)

The pH may be measured with a meter or calculated if certain parameters are established. Water itself dissociates to a small extent to produce equal quantities of H⁺ and OH^- ions displayed in the following equilibrium:

pH , originally defined by Danish biochemist Søren Peter Lauritz Sørensen in 1909, is a measure of the concentration of hydrogen ions. The term pH was derived from the manner in which the hydrogen ion concentration is calculated, it is the negative logarithm of the hydrogen ion (H⁺) concentration:

where log is a base-10 logarithm and a_H+ is the activity (related to concentration) of hydrogen ions. The "p" in Equation stands for the German word for "power", potenz, so pH is an abbreviation for "power of hydrogen".

A higher pH means there are fewer free hydrogen ions, and that a change of one pH unit reflects a tenfold change in the concentrations of the hydrogen ion. For example, there are 10 times as many hydrogen ions available at pH 7 than at pH 8. The pH scale commonly quoted ranges from 0 to 14 with a pH of 7 considered to be neutral.

Substances with a pH less that 7 are considered to be acidic and substances with pH equal to or greater than 7 to be basic or alkaline. Thus, a pH of 2 is very acidic and a pH of 12 very alkaline. However, it is technically possible to have very acidic solutions with a pH lower than zero and concentrated caustic solutions with a pH greater than 14. Such solutions are in fact typical of many ore extracting processes that require the digestive power of caustics and acids.

Low pH acid waters accelerate corrosion by supplying hydrogen ions to the corrosion process. Although even absolutely pure water contains some free hydrogen ions, dissolved carbon dioxide (CO₂) in the water can increase the hydrogen ion concentration. Dissolved CO₂ may react with water to form carbonic acid (H₂CO₃) as shown in equation.

where K_eq is the reaction equilibrium expressed as a ratio.

Carbonic acid subsequently dissociates in bicarbonate and carbonate ions as expressed respectively in the following equations:

Care must be taken when quoting and using the dissociation constant in equation. This equilibrium value is correct for the H₂CO₃ molecule, and shows that it is a stronger acid than acetic acid or formic acid as might be expected from the influence of the electronegative oxygen substituent. However, carbonic acid only exists in solution in equilibrium with carbon dioxide, and so the concentration of H₂CO₃ is much lower than the concentration of CO₂, reducing the measured acidity. The equation may be rewritten as follows:

Even more acidity is sometimes encountered in mine waters and in water contaminated by industrial wastes. Many salts added to an aqueous system also have a direct effect on the pH of that mixture through the following process of hydrolysis shown here for the addition of ferric ions to water:

In this particular example the equilibrium is established between ferric ions, water, ferric hydroxide or Fe(OH)₃ and the acidity of the water. This particular example is quite useful to explain the severity of a situation that can develop in confined areas such as corrosion pitting and crevices.

BRAZING PROCESSES

Generally, brazing processes are specified according to heating methods (sources) of industrial significance. Whatever the process used, the filler metal has a melting point above 840°F (450°C) but below the base metal and distributed in the joint by capillary attraction. The brazing processes are:

(a) Torch brazing.

(b) Furnace brazing.

(c) Induction brazing.

(d) Resistance brazing.

(e) Dip brazing.

(f) Infrared brazing.

(a) Torch brazing.

(a) Torch brazing tip size, filler metal of is performed by heating with a gas torch with a proper required composition, and appropriate flux. This depends on the temperature and heat amount required. The fuel gas (acetylene, propane, city gas, etc.) may be burned with air, compressed air, or oxygen.

(b) Brazing filler metal may be preplaced at the joint in the forms of rings, washers, strips, slugs, or powder, or it may be fed from hand-held filler metal in wire or rod form. In any case, proper cleaning and fluxing are essential.

(c) For manual torch brazing, the torch may be equipped with a single tip, either single or multiple flame. Manual torch brazing is particularly useful on assemblies involving sections of unequal mass. Welding machine operations can be set up where the production rate allows, using one or several torches equipped with single or multiple flame tips. The machine may be designed to move either the work or torches, or both. For premixed city gas-air flames, a refractory type burner is used.

(b) Furnace brazing.

(a) Furnace brazing is used extensively where the parts to be brazed can be assembled with the brazing filler metal in form of wire, foil, filings, slugs, powder, paste, or tape is preplaced near or in the joint. This process is particularly applicable for high production brazing. Fluxing is employed except when an atmosphere is specifically introduced in the furnace to perform the same function. Most of the high production brazing is done in a reducing gas atmosphere, such as hydrogen and combusted gases that are either exothermic (formed with heat evolution) or endothermic (formed with heat absorption). Pure inert gases, such as argon or helium, are used to obtain special atmospheric properties.

(b) A large volume of furnace brazing is performed in a vacuum, which prevents oxidation and often eliminates the need for flux. Vacuum brazing is widely used in the aerospace and nuclear fields, where reactive metals are joined or where entrapped fluxes would be intolerable. If the vacuum is maintained by continuous pumping, it will remove volatile constituents liberated during brazing. There are several base metals and filler metals that should not be brazed in a vacuum because low boiling point or high vapor pressure constituents may be lost. The types of furnaces generally used are either batch or contiguous. These furnaces are usually heated by electrical resistance elements, gas or oil, and should have automatic time and temperature controls. Cooling is sometimes accomplished by cooling chambers, which either are placed over the hot retort or are an integral part of the furnace design. Forced atmosphere injection is another method of cooling. Parts may be placed in the furnace singly, in batches, or on a continuous conveyor.

(c) Vacuum is a relatively economical method of providing an accurately controlled brazing atmosphere. Vacuum provides the surface cleanliness needed for good wetting and flow of filler metals without the use of fluxes. Base metals containing chromium and silicon can be easily vacuum brazed where a very pure, low dew point atmosphere gas would otherwise be required.

(c) Induction brazing.

(a) In this process, the heat necessary to braze metals is obtained from a high frequency electric current consisting of a motor-generator, resonant spark gap, and vacuum tube oscillator. It is induced or produced without magnetic or electric contact in the parts (metals). The parts are placed in or near a water-cooled coil carrying alternating current. They do not form any part of the electrical circuit. The brazing filler metal normally is preplaced.

(b) Careful design of the joint and the coil setup are necessary to assure that the surfaces of all members of the joint reach the brazing temperature at the same time. Flux is employed except when an atmosphere is specifically introduced to perform the same function.

(c) The equipment consists of tongs or clamps with the electrodes attached at the end of each arm. The tongs should preferably be water-cooled to avoid overheating. The arms are current carrying conductors attached by leads to a transformer. Direct current may be used but is comparatively expensive. Resistance welding machines are also used. The electrodes may be carbon, graphite, refractory metals, or copper alloys according to the required conductivity.

(d) Resistance brazing. The heat necessary for resistance brazing is obtained from the resistance to the flow of an electric current through the electrodes and the joint to be brazed. The parts comprising the joint form a part of the electric circuit. The brazing filler metal, in some convenient form, is preplaced or face fed. Fluxing is done with due attention to the conductivity of the fluxes. (Most fluxes are insulators when dry.) Flux is employed except when an atmosphere is specifically introduced to perform the same function. The parts to be brazed are held between two electrodes, and proper pressure and current are applied. The pressure should be maintained until the joint has solidified. In some cases, both electrodes may be located on the same side of the joint with a suitable backing to maintain the required pressure.

(e) Dip brazing.

(a) There are two methods of dip brazing: chemical bath dip brazing and molten metal bath dip brazing.

(b) In chemical bath dip brazing, the brazing filler metal, in suitable form, is preplaced and the assembly is immersed in a bath of molten salt. The salt bath furnishes the heat necessary for brazing and usually provides the necessary protection from oxidation; if not, a suitable flux should be used. The salt bath is contained in a metal or other suitable pot, also called the furnace, which is heated from the outside through the wall of the pot, by means of electrical resistance units placed in the bath, or by the I²R loss in the bath itself.

(c) In molten metal bath dip brazing, the parts are immersed in a bath of molten brazing filler metal contained in a suitable pot. The parts must be cleaned and fluxed if necessary. A cover of flux should be maintained over the molten bath to protect it from oxidation. This method is largely confined to brazing small parts, such as wires or narrow strips of metal. The ends of the wires or parts must be held firmly together when they are removed from the bath until the brazing filler metal has fully solidified.

(f) Infrared brazing.

(a) Infrared heat is radiant heat obtained below the red rays in the spectrum. While with every "black" source there is sane visible light, the principal heating is done by the invisible radiation. Heat sources (lamps) capable of delivering up to 5000 watts of radiant energy are commercially available. The lamps do not necessarily need to follow the contour of the part to be heated even though the heat input varies inversely as the square of the distance from the source. Reflectors are used to concentrate the heat.

(b) Assemblies to be brazed are supported in a position that enables the energy to impinge on the part. In some applications, only the assembly itself is enclosed. There are, however, applications where the assembly and the lamps are placed in a bell jar or retort that can be evacuated, or in which an inert gas atmosphere can be maintained. The assembly is then heated to a controlled temperature, as indicated by thermocouples. The part is moved to the cooling platens after brazing.

(g) Special processes.

(a) Blanket brazing is another of the processes used for brazing. A blanket is resistance heated, and most of the heat is transferred to the parts by two methods, conduction and radiation, the latter being responsible for the majority of the heat transfer.

(b) Exothermic brazing is another special process by which the heat required to melt and flow a commercial filler metal is generated by a solid state exothermic chemical reaction. An exothermic chemical reaction is defined as any reaction between two or more reactants in which heat is given off due to the free energy of the system. Nature has provided us with countless numbers of these reactions; however, only the solid state or nearly solid state metal-metal oxide reactions are suitable for use in exothermic brazing units. Exothermic brazing utilizes simplified tooling and equipment. The process employs the reaction heat in bringing adjoining or nearby metal interfaces to a temperature where preplaced brazing filler metal will melt and wet the metal interface surfaces. The brazing filler metal can be a commercially available one having suitable melting and flow temperatures. The only limitations may be the thickness of the metal that must be heated through and the effects of this heat, or any previous heat treatment, on the metal properties.

BRAZING

a. General.

(1) Brazing is a group of welding processes which produces coalescence of materials by heating to a suitable temperature and using a filler metal having a liquidus above 840°F (449°C) and below the solidus of the base metals. The filler metal is distributed between the closely fitted surfaces of the joint by capillary attraction. Brazing is distinguished from soldering in that soldering employs a filler metal having a liquidus below 840°F (449°C).

(2) When brazing with silver alloy filler metals (silver soldering), the alloys have liquidus temperatures above 840°F (449°C).

(3) Brazing must meet each of three criteria:

(a) The parts must be joined without melting the base metals.

(b) The filler metal must have a liquidus temperature above 840°F (449°C).

(c) The filler metal must wet the base metal surfaces and be drawn onto or held in the joint by capillary attraction.

(4) Brazing is not the same as braze welding, which uses a brazing filler metal that is melted and deposited in fillets and grooves exactly at the points it is to be used. The brazing filler metal also is distributed by capillary action. Limited base metal fusion may occur in braze welding.

(5) To achieve a good joint using any of the various brazing processes, the parts must be properly cleaned and protected by either flux or the atmosphere during heating to prevent excessive oxidation. The parts must provide a capillary for the filler metal when properly aligned, and a heating process must be selected that will provide proper brazing temperatures and heat distribution.

b. Principles.

(1) Capillary flow is the most important physical principle which ensures good brazements providing both adjoining surfaces molten filler metal. The joint must also be properly spaced to permit efficient capillary action and resulting coalescence. More specifically, capillarity is a result of surface tension between base metal(s), filler metal, flux or atmosphere, and the contact angle between base and filler metals. In actual practice, brazing filler metal flow characteristics are also influenced by considerations involving fluidity, viscosity, vapor pressure, gravity, and by the effects of any metallurgical reactions between the filler and base metals.

(2) The brazed joint, in general, is one of a relatively large area and very small thickness. In the simplest application of the process, the surfaces to be joined are cleaned to remove contaminants and oxide. Next, they are coated with flux or a material capable of dissolving solid metal oxides present and preventing new oxidation. The joint area is then heated until the flux melts and cleans the base metals, which are protected against further oxidation by the liquid flux layer.

(3) Brazing filler metal is then melted at some point on the surface of the joint area. Capillary attraction is much higher between the base and filler metals than that between the base metal and flux. Therefore, the flux is removed by the filler metal. The joint, upon cooling to room temperature, will be filled with solid filler metal. The solid flux will be found on the joint surface.

(4) High fluidity is a desirable characteristic of brazing filler metal because capillary attraction may be insufficient to cause a viscous filler metal to run into tight fitting joints.

(5) Brazing is sometimes done with an active gas, such as hydrogen, or in an inert gas or vacuum. Atmosphere brazing eliminates the necessity for post cleaning and ensures absence of corrosive mineral flux residue. Carbon steels, stainless steels, and super alloy components are widely processed in atmospheres of reacted gases, dry hydrogen, dissociated ammonia, argon, and vacuum. Large vacuum furnaces are used to braze zirconium, titanium, stainless steels, and the refractory metals. With good processing procedures, aluminum alloys can also be vacuum furnace brazed with excellent results.

(6) Brazing is a process preferred for making high strength metallurgical bonds and preserving needed base metal properties because it is economical.

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Brazing Filler Metals

For satisfactory use in brazing applications, brazing filler metals must possess the following properties:

(1) The ability to form brazed joints possessing suitable mechanical and physical properties for the intended service application.

(2) A melting point or melting range compatible with the base metals being joined and sufficient fluidity at brazing temperature to flow and distribute into properly prepared joints by capillary action.

(3) A composition of sufficient homogeneity and stability to minimize separation of constituents (liquation) under the brazing conditions to be encountered.

(4) The ability to wet the surfaces of the base metals being joined and form a strong, sound bond.

(5) Depending on the requirements, ability to produce or avoid base metal-filler metal interactions.

BRAZING GRAY CAST IRON

a. Gray cast iron can be brazed with very little or no preheating. For this reason, broken castings that would otherwise need to be dismantled and preheated can be brazed in place. A nonferrous filler metal such as naval brass (60 percent copper, 39.25 percent zinc, 0.75 percent tin) is satisfactory for this purpose. This melting point of the nonferrous filler metal is several hundred degrees lower than the cast iron; consequently the work can be accomplished with a lower heat input, the deposition of metal is greater and the brazing can be accomplished faster. Because of the lower heat required for brazing, the thermal stresses developed are less severe and stress relief heat treatment is usually not required.

b. The preparation of large castings for brazing is much like that required for welding with cast iron rods. The joint to be brazed must be clean and the part must be sufficiently warm to prevent chilling of filler metal before sufficient penetration and bonding are obtained. When possible, the joint should be brazed from both sides to ensure uniform strength throughout the weld. In heavy sections, the edges should be beveled to form a 60 to 90 degree V.

Corrosion in Acids

One of the common ways of generating hydrogen in a laboratory is to place zinc into a dilute acid, such as hydrochloric or sulfuric. When this is done, there is a rapid reaction in which the zinc is attacked or “dissolved” and hydrogen is evolved as a gas.

Rapid evolution of hydrogen bubbles during the corrosion of a zinc strip in a 1 M HCl acid solution

These reactions are described in the following equations to:

These equations are the chemical shorthand for the statement: One zinc atom + two hydrochloric acid molecules dissociated as ions H⁺ and Cl^- and becomes one molecule of zinc chloride in the first equation and written as a soluble salt in the form of Zn²⁺ and Cl^- ions in the second equation + one molecule of hydrogen gas which is given off as indicated by the vertical arrow. It should be noted that the chloride ions do not participate directly in this reaction, although they could play an important role in real corrosion situations.
Similarly, zinc combines with sulfuric acid to form zinc sulfate (a salt) and hydrogen gas as shown in the following equations:

Note that each atom of a substance that appears on the left-hand side of these equations must also appear on the right-hand side. There are also some rules that denote in what proportion different atoms combine with each other. As in the preceding reaction, the sulfate ions that are an integral part of sulfuric acid do not participate directly to the corrosion attack and therefore one could write these equations in a simpler form:

Many other metals are also corroded by acids often yielding soluble salts and hydrogen gas as shown in Equations and for respectively iron and aluminum:

Note that zinc and iron react with two H⁺ ions, whereas aluminum reacts with three. This is due to the fact that both zinc and iron, when corroding, each lose two electrons and display two positive charges in their ionic form. They are said to have a valence of +2 or II, whereas aluminum loses three electrons when leaving an anodic surface and hence displays three positive charges and is said to have a valence of +3 or III. Some metals have several common valences, others only one. The following Figure shows Some of the oxidation states found in compounds of the transition-metal elements.

Oxidation states found in compounds of the metalic elements. A solid circle represents a common oxidation state, and a ring represents a less common (less energetically favorable) oxidation state