Impact Toughness Testing of Metals...

Impact Toughness Testing of Metals Metals undergo dynamic fracture under rapidly applied loads which are generally produced by impact or by explosive detonation. In comparison to quasi-static loading, dynamic conditions involve loading rates which are higher than those encountered in conventional tensile testing or fracture mechanics testing. Dynamic fracture includes two cases namely (i) a stationary crack subjected to a rapidly applied load, and (ii) a rapidly propagating crack under a quasi-stationary load. In both the cases the material at the crack tip is strained rapidly and, if rate sensitive, can offer less resistance to fracture than at quasi-static strain rates. As an example, values for dynamic fracture toughness are lower than those for static toughness as experienced in the testing of low carbon steels at different temperatures. Many structural components are subjected to high loading rates in service. They also are to survive high loading rates during accident conditions. This makes high strain rate fracture testing is of interest and components are to be designed against crack initiation under high loading rates or designed to arrest a rapidly running crack. Also, since dynamic fracture toughness is normally lower than static toughness, more conservative analysis requires consideration of dynamic toughness. Measurement and analysis of fracture behaviour under high loading rates is more complex than under quasi-static conditions. There are also several different test methods which are used in the evaluation of dynamic fracture resistance. Test methods based on fracture mechanics produce quantitative values of fracture toughness parameters which are useful in design. However, several qualitative methods are also been used in the evaluation of impact energy to break a notched bar, percent of cleavage area on fracture surfaces, or the temperature for nil ductility or crack arrest. These qualitative tests include methods such as...

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...

Nitrogen in Steels May23

Nitrogen in Steels

Nitrogen in Steels All steels contain some nitrogen which can enter the steel as an impurity or as an intentional alloying addition. The quantity of nitrogen in steels normally depends on the residual level arising from the steelmaking processes or the amount aimed in case of deliberate addition. There are significant differences in residual levels of nitrogen in steels produced from the two main steelmaking processes. Basic oxygen furnace (BOF) process generally results into lower residual nitrogen in steels, typically in the range of 30 to 70 ppm while electric arc furnace (EAF) process results into higher residual nitrogen, typically in the range of 70 to 110 ppm. Nitrogen is added to some steels (e.g. steels containing vanadium) to provide sufficient nitrogen for formation of nitride to achieve higher strength. In such steels nitrogen levels can increase to 200 ppm or higher. Nitrogen in the liquid steel is present in the form of solution. During the solidification of steel in continuous casting, three nitrogen related phenomena can happen. These are Formation of blow holes Precipitation of one or more nitride compounds Solidification of nitrogen in interstitial solid solution. The maximum solubility of nitrogen in liquid iron is around 450 ppm, and less than 10 ppm at ambient temperature (Fig 1). The presence of significant quantities of other elements in liquid iron affects the solubility of nitrogen. Mainly the presence of dissolved sulfur and oxygen limit the absorption of nitrogen because they are surface active elements. Fig 1 Solubility of nitrogen in iron Nitrogen is generally considered as undesirable impurity which causes embrittlement in steels and affects strain aging. However nitrogen produces a marked (intersititial solid solution) strengthening when diffused into the surface of the steel, similar to the strengthening observed during case hardening (Nitriding)....