Welding of Carbon and Low Alloy Steels and Hydrogen Induced Cracking Sep23

Welding of Carbon and Low Alloy Steels and Hydrogen Induced Cracking...

Welding of Carbon and Low Alloy Steels and Hydrogen Induced Cracking Arc welding is a process by which steels are joined by coalescence. Normally the process uses a compatible filler material. Before a well-bonded joint is produced, the joint surface is to be heated above the melting temperature in order to completely fuse with the weld metal. Though the metallurgical reactions which involve melting, solidification, and solid-state transformation are not unusual, the temperatures and cooling rates observed are severe. Active gases also are present and can dissolve in the fused steel. Fluxes are introduced to alloy with and protect the weld metal. Generally, joints are rigid and restrain dimensional changes caused by shrinkage and solid-state transformations, producing residual stresses of yield-strength (YS) magnitude. Since the metallurgical changes do not occur under equilibrium conditions, and since the stresses are high, many of the reactions can take place in either or both the weld metal and the heat affected zone (HAZ) of the steel and can produce defects that weaken their soundness. Because of the tremendous variability of the welding processes, it is difficult to provide much detail about the exact mechanisms involved or the corrections that can be made. Furthermore, many corrective measures are obvious once most defects are explained. One problem, which relates to hydrogen (H2), is not simple. Since this problem is becoming more relevant as more high-strength, low-alloy (HSLA) steels are being welded, the subject of hydrogen-induced cracking (HIC) is very important. Carbon (C) and low alloy steels are welded since they have widespread application and good weldability. This usefulness is mainly due to the metallurgical characteristics of the iron (Fe) base system. The characteristic includes the ability to undergo allotropic (microstructural) transformation which allows the opportunity for hardening and strengthening through...

Steels for Shipbuilding...

Steels for Shipbuilding Ship structures are determined by the ship’s mission and intended service. These determine a ship’s size, complexity and the function of the structural components. There are inherent uncertainties in the loads imposed on the ship structure because of the random nature of the loads imposed by the marine environment. Unlike a fixed, land-based structure, a ship derives its entire support from the buoyancy provided by a fluid, which transmits these loads to the hull structure. Iron hulls replaced wooden hulls in the second half of the 18th century, to be followed up by steel. Since then seagoing ships and inland barges are being regularly designed with several steel grades and shapes. Steels are the most common materials being used for shipbuilding. These steels are rather to meet strict requirements such as strength, flexibility, high manufacturability, weldability, and cost, reparability, etc.  Steels used in the shipbuilding industry also need high cold-resistance, good welding characteristics and increased fracture strength. Modern steel shipbuilding involves the fabrication of a complex steel structure, into which a wide range of ready-made equipment is fixed. Today the principal raw material is steel plate and the layout of  a modern shipyard is arranged to facilitate the flow of steel received from the steel plant through the various processes of making out, cutting, bending, welding, fabricating subassemblies, and final erection of the prefabricated units into the hull and the superstructure. In shipbuilding, there is usually a trade off in the use of material and complex structures. Typically, a complex structure requires more labour and fabrication than a simpler structure, which uses more material. There is also a tradeoff between using more complex structure and the lighter weight of the vessel, as a lighter ship can carry more cargo for a...

Titanium in Steels

Titanium in Steels  Titanium (Ti) (atomic number 22 and atomic weight 47.90) has density of 4.52 gm/cc. Melting point of Ti is 1660 deg C and boiling point is 3287 deg C. Ti is a highly active element. It usually forms a stable oxide coating at room temperature on its surface, which limits further oxidation. The phase diagram of the Fe (iron)-Ti binary system is at Fig 1. Fig 1 Fe-Ti phase diagram Ti forms stable compounds with oxygen (O), carbon (C), nitrogen (N) and sulfur (S) at temperatures of steelmaking. It is sometimes used in steelmaking because of its property for fixing of these elements in order to reduce their harmful effects. Ti is also used for the purpose of grain refining in many steels. In many respects, functions of Ti are similar to the addition of both aluminum (Al) and niobium (Nb). Ti is more expensive than Al; hence it is rarely used as a deoxidizer.  The reactivity of Ti is similar to that of magnesium (Mg) and it can quite easily be set on fire. It burns with a bright white flame, which can be harmful to look at. Ferrotitanium powder is also flammable, with the powder having finer size and higher Ti content being more hazardous. Ti ores are mainly ilmenite (FeO.TiO2) and rutile (TiO2).  Addition agents Ti containing addition agents are Ti metal scrap, ferroalloys and master alloys. Ti metal scrap may be of commercial purity Ti. Ti metal scrap is of two types one with 6 % Al and 4 % vanadium (V) while the second with 6 % Al, 2 % tin (Sn), 4 % zirconium (Zr), and 2 % molybdenum (Mo). Sn is usually an unwanted element in steels. Since the melting point of Ti is...

Niobium in Steels

Niobium in Steels  Niobium (Nb) (atomic number 41 and atomic weight 92.91) has density of 8.57 gm/cc. The element is also known as Columbium (Cb). Melting point of Nb is 2467 deg C and boiling point is 4740 deg C. Cb was discovered in 1801 by Charles Hatchett, who named the element to signify the American origin. This name was used in USA while Europe used the name niobium for the same element. To end this confusion, the name niobium was chosen for element 41 at the 15th Conference of the Union of Chemistry in Amsterdam in 1949. A year later this name was officially adopted by the International Union of Pure and Applied Chemistry (IUPAC) after 100 years of controversy, despite the chronological precedence of the name Columbium. Columbium name is still used in many places in USA. The phase diagram of the Fe-Nb binary system is at Fig 1. Fig 1 Fe-Nb phase diagram  The use of Nb dates back to 1925 when it was used to replace tungsten (W) in tool steel production. By the 1930s, Nb was being used to prevent corrosion in stainless steels. The ability of Nb to maintain fine grain sizes in steels at higher temperatures has been known since 1940s and steels that take advantage of this effect have been commercially produced for many years. In recent years, however, Nb is being known more as one of the most important element for the micro alloying. Nb plays an important role in HSLA (high strength low alloy) steels. Nb also has important applications in tool steels, wear and abrasion resistant steels, steels for high temperature service, stainless steels and super alloys. Many of these uses depend on the strong affinity of Nb for carbon (C) and/or nitrogen (N). Addition agents...

Nickel in Steels

Nickel in Steels  Nickel (Ni) (atomic number 28 and atomic weight 58.69) has density of 8.902 gm/cc. Melting point of Ni is 1455 deg C and boiling point is 2910 deg C. The phase diagram of the Fe-Ni binary system is at Fig 1. Ni has a face centered cubic (f.c.c.) crystal structure. It is ferromagnetic up to 353 deg C, its curie point.   Fig 1 Fe-Ni phase diagram Ni is an important and widely used constituent of alloy steels. It is best known as a solid solution strengthener, a mild hardenability agent and, most important, as a means of promoting high toughness, especially at low temperatures. Ni is an important ingredient in stainless steel, helping it to prevent rust, scratches and resist heat. Around 65 % of global Ni production goes into the production of stainless steel. Ni alloyed steels contain as little as fraction of a percent to almost 30 % Ni. As may be expected, properties of these alloy steels range from strengths similar to plain carbon steel to some of the strongest metallic materials known. On the lower side of the Ni percentage in the steels are the alloy and HSLA (high strength and low alloy) structural steels. Hot rolled steels with yield strengths of 345 MPa may contain 0.50 % to 2.00 % Ni for toughness and added corrosion resistance. Age hardening steels contain 1.3 % to 1.5 % Ni plus copper (Cu) and niobium (Nb). Quenched and tempered or normalized and tempered structural steels contain nickel (Ni) up to 2.25 %, as well as a variety of other constituents including chromium (Cr), molybdenum (Mo) or boron (B). Nickel bearing addition agents Ni bearing addition agents are ferro- nickel (Fe- Ni) ferroalloy, Ni containing steel scrap, Nickel oxide...