Offsetting shrinkage in ductile iron what thermal analysis shows by david sparkman may 30, 2001 last revision february 11 th 2006 abstract the natural shrinkage that occurs during the solidification of ductile iron can be. The high temperatures at which annealing occurs serve to. In this case, the metal behaves in a ductile manner. The ductile iron subsequently cools and solidifies in the shape of the void it occupies. Methods and cycle times the following cycles are guidelines for each of the most commonly used heat treat methods. A stress concentration does not allow microscopic movements, so brittle fracture is more likely. These cycles are applicable to standard durabar grades. In most cases, inmold stress relief is sufficient to obtain a stable structure. This sae standard covers the minimum mechanical properties measured on separately cast test pieces of varying thickness and microstructural requirements for ductile iron castings used in. The distinct, indisputable advantages to using ductile iron pipe instead of steel pipe have long been recognized by engineers, contractors, construction firm owners, and community leaders. Some suppliers do not stress relieve gray iron castings, and some do. In regions where there is no stress concentration, it is easier for microscopic movements to occur. This would remove harmful residual stresses that could result in failure at thickness transitions in service, or distortion during machining.
Information concerning welding ductileiron, is offered in this page. Some of the most popular heattreating techniques include carburizing, nitriding, hardening and tempering, annealing and stress relief, and cryogenic hardening. Table 1 comparison of ductile iron pipe and pvc pipe standards ductile iron pipe versus pvc 2 topic ductile iron pipe ansiawwa c150a21. National standard for the thickness design of ductile iron pipe ansiawwa c150a21.
Considering the thickness variations that you described, i would recommend a stress relief of the casting between to 1100 deg f. Ductile iron castings are solid metal objects that result from molten ductile iron being poured into voids in a mold. Stress relieving is a general term in heat treating, describing a wide range of processes. In ferrous heat treatment, the critical temperature al is the temperature above which the austenite phase is stable. In addition, the potential for stress corrosion cracking is reduced, and the metallurgical structure can be improved through stress relieving. Stress relieving gray cast iron practical machinist. A tightened bolt applies shear stress on the threads. Sae j434 for automotive ductile nodular iron castings. All ductile iron heat treatments, apart from stress relief, tempering and subcritical annealing, involve heating the casting to a temperature above the critical temperature range figure 7. Ductile iron pipe offers more durability, a higher degree of safety, an extremely.
Heat treatment of ductile cast iron includes stress relief and annealing, as well as heat treatments used for steels including normalize and temper for higher strength and wearresistance, quench and temper for the highest strength, and. Silicon influences the room temperature mechanical properties of ductile iron through solid solution hardening of the ferrite matrix. Stress relieving, a lowtemperature treatment, to reduce or relieve internal stresses remaining after casting. Circumferentially notched tension specimens were used, and failure initiation strains were correlated with a parameter which is a measure of the triaxiality of the stress state. The following is a typical stress relieving treatment employed by our foundry for our gray iron castings 24 hours full cycle, with about 68 hours of soak at approximately 550 c. Size thin parts are more likely to fail in a ductile manner when overloaded. Conventional stress relieving applied to carbonlow alloy steels and cast iron is typically carried out between 500c and 680c. One set of experiments consisted of making relaxation tests and using the. Nodulizing elements, typically magnesium, are used to allow the solidification of the graphite into nodules.
Annealed cast ductile iron can be bent, twisted or deformed without fracturing. Durabar ductile iron shear strength a vertical load applied on a part similar to the way scissors cut through paper creates shear stress. In this article, were not referring to gating design, casting geometry or casting procedures. Ductile iron what ductile iron is ductile iron is a family of cast graphitic irons which possess high strength, ductility and resistance to shock. These processes are used to improve various properties of metal, including strength, abrasion resistance, corrosion resistance, durability, and hardness, among others. The effect of tempering temperature and time on different mechanical properties of ductile cast iron. Stress relieving is a process used for reducing or eliminating stresses in iron castings by reheating and cooling them at a controlled rate. The importance of stress relieving in gray iron castings. This is a special class of casting materials of this name. Effect of tempering temperature and time on strength and. The ferrous metals, steel or cast irons are essentially an alloy of iron and carbon.
The exact postweld heat treatment of welded cast iron depends on the degree of stress relief or softening required. In ferrous heat treatment, the critical temperature a l is the temperature above which the austenite phase is stable. Typically, it is best to stress relieve after rough machining and just prior to final machining. Heat treatment of cast irons sukomal ghosh national metallurgical laboratory. If you dont put them in, you dont have to take them out.
Heat treatments of ductile cast iron include stress relief and annealing, and heat treatments used for steels include normalize and temper for higher strength and wear resistance, quench and temper for the highest strength and austempering. The purpose is to findthe effect of varying parameters and compare the result. Stress relief annealing does not involve major microstructural transformations, whereas. Invented in 1943, ductile iron is a modern iteration of cast iron. Ductile iron castings are not as section sensitive as gray iron. What is shrinkage casting defects potential in the melt. Annealing ductile cast iron when maximum ductility and good machinability are desired and high strength is not required, ductile iron castings are generally given a full ferritizing anneal.
The steel becomes softer and more ductile through the precipitation of iron carbide at temperatures associated with stress relieving. S design stress psi ductile iron pipe versus pvc 1 ductile iron pipe versus pvc t pd 2s. Other heat treatments in common industrial use include stress relief annealing and selective surface heat treatment. Design basis the basis of the design standard for ductile iron pipe is the long established fact that ductile iron. No more thorough and comprehensive standard design procedure exists for any piping material. Ductile iron, like cast iron in gen eral, represents a large group of ma terials offering a wide range of prop erties.
Sae j434 for automotive ductile nodular iron castings by dandong foundry in china. The sleeves are spun cast molten iron is poured into a spinning mold one at a time to remove impurities and provide a denser casting. The microstructure is thus converted to ferrite, and the excess carbon is deposited on the existing nodules. Figure 2 classifies cast irons according to their commercial names, applications, and structures. In practice and industry, this reduction of gibbs free energy is termed stress relief.
Nodular cast irons or ductile, or spheroidal graphite iron are primarily heat treated. However, annealing lowers the ascast tensile strength of all but the softest irons. The effect of stress state on the effective plastic strain to initiate ductile failure in three high strength steels is investigated. Weldingductileiron for increased versatility of use. The latter are generated by austempering, an isothermal reaction process resulting in a product called austempered ductile iron adi. On the influence of state of stress on ductile failure. Generally, metal stress relieving involves heating a part to a temperature at which the yield strength is sufficiently low to the point which internal stresses can relieve themselves. A separate page was dedicated to cast iron welding. Otherwise, for most of the applicaitons where stress risers are a problem, you have to use ductile iron. David sparkman at meltlab presented this theory at the ductile iron society last october 2015. However, ductile iron has very poor weldability in comparison to. Vibratory stress relief, often abbreviated vsr, is a nonthermal stress relief method used by the metal working industry to enhance the dimensional stability and mechanical integrity of castings, forgings, and welded components, chiefly for two categories of these metal workpieces. In ductile iron, shear strength is 90% of tensile, and in steel, shear strength is only 50% of tensile.
The article elucidates the effects of these heat treatments on the mechanical properties of the ductile irons. Conditions for altering softening and for stress relief are described for grey irons and ductile irons. Further, the results clearly indicate that the relevant standards 10, 18 for ductile niresists do not provide the required protection against scc in marine service even after subjecting the cast materials to suitable stress relief heat treatments, again in contrary to what was reported above. Its strength, toughness and ductility duplicate many grades of steel and far exceed those of standard gray irons. Heat treating guide continuous cast iron bar stock. Stress relieving, a lowtemperature treatment, to reduce or relieve internal. Nodular cast iron, also referred to as ductile iron or spheroidal graphite iron, is a group of irons in which the graphite forms as nodules spherical instead of flakes as in normal cast iron. Stress corrosion cracking of ductile niresist irons and. Heat treatment of cast irons 20181212 industrial heating. For other common applications, austenitic cast irons are stress relieved at. High stress in castings can cause distortion or warping especially during machining.
It discusses the most important heat treatments of ductile irons, namely, stress relieving, austenitizing, annealing, normalizing, quenching, martempering, austempering, and surface hardening. All ductile iron heat treatments, apart from stress relief, tempering and subcritical annealing, involve heating the casting to a. Ductile iron for open gearing a current perspective. Heat treatments of ductile cast iron include stress relief and annealing, and heat treatments used for steels include normalize and temper for. All ductile iron heat treatments, apart from stress relief, tempering and subcritical annealing, involve heating the casting to a temperature above the critical temperature range. The objective of stress relieving is to remove the residual stresses developed during solidification or machining operation or a combination of both. As a rule, sandmark stress relieves all our gray iron castings. Aufhauser technical guide cast iron welding procedures. The figure 25 graph shows the stress relief obtained in ductile iron held at three temperatures for half an hour to eight hours. Stress relieving iron castings practical machinist. Common reasons for presence of stresses in castings are.
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