7 Alloy Steels
At the end of this lesson students should be able to: Classify alloy steels Explain: effects of alloying elements to steel properties Discuss: composition, microstructure, mechanical properties and engineering applications of various type of alloy steels Select : suitable steel for appropriate purposes 7-2
Classification of Ferrous Alloys Metal Alloys Ferrous Nonferrous Steels Cast Irons Low Alloy Gray iron Ductile iron White iron Malleable iron Low-carbon Medium-carbon High-carbon High Alloy Plain High strength, low alloy Plain Heat treatable Plain Tool Stainless 7-3
Classification of Ferrous Alloys Based on carbon content Pure iron (< 0.008wt% C) From the phase diagram, it is composed almost exclusively of the ferrite phase at room temperature. Steels (0.008 ~ 2.14wt% C) In most steels the microstructure consists of both a and Fe 3 C phases. Carbon concentrations in commercial steels rarely exceed 1.0 wt%. Cast irons (2.14 ~ 6.70wt% C) Commercial cast irons normally contain less than 4.5wt% C 7-4
The carbon content is normally less than 1.0 wt%. Plain carbon steels: containing only residual concentrations of impurities other than carbon and a little manganese About 90% of all steel made is carbon steel. Alloy steels: more alloying elements are intentionally added in specific concentrations. Stainless steels Ferrous Alloys Steels 7-5
Classification of Steels According to Their Carbon Contents Low-carbon steels Less than 0.25 wt%c Medium-carbon steels 0.25 ~ 0.60 wt%c High-carbon steels 0.60 ~ 1.4 wt%c 7-6
A four-digit number: The Designation of Steels the first two digits indicate the alloy content; the last two, the carbon concentration For plain carbon steels, the first two digits are 1 and 0; alloy steels are designated by other initial two-digit combinations (e.g., 13, 41, 43) The third and fourth digits represent the weight percent carbon multiplied by 100 For example, a 1040 steel is a plain carbon steel containing 0.40 wt% C. 7-7
The Designation of Steels A four-digit number: the first two digits indicate the alloy content; the last two, the carbon concentration 41 40 Identifies major alloying element(s) Percentage of carbon 7-8
Table 11.2a AISI/SAE and UNS Designation Systems AISI: American Iron and Steel Institute SAE: Society of Automotive Engineers Chapter UNS: 7 Uniform Numbering System 7-9
94XX N i- Steels Steel Numerical Name 10XX, 11 XX 13XX 23XX, 25 XX 31XX, 33XX, 303XX 40XX 41XX 43XX & 47XX 44XX 48XX 50XX, 51XX, 501XX, 521XX, 61XX 81XX, 86XX, 87XX, 88XX 92XX 93XX, 98XX 94XX XXBXX XXLXX Key Alloys Carbon only Manganese Nickel Nickel-Chromium Mo Cr-Mo Ni-Cr-Mo Mn-Mo Ni-Mo Cr Cr-V Ni-Cr-Mo Si-Mn Ni-Cr-Mo Ni-Cr-Mo-Mn Boron Lead 7-10
Low-Carbon Steels Less than 0.25 wt%c Unresponsive to heat treatments intended to form martensite; strengthening is accomplished by cold work Microstructures: ferrite and pearlite Relatively soft and weak, but having outstanding ductility and toughness Typically, s y = 275 MPa, s UT = 415~550 MPa, and ductility = 25%EL Machinable, weldable, and, of all steels, are the least expensive to produce Applications: automobile body components, structural shapes, and sheets used in pipelines, buildings, bridges, etc. 7-11
TTT Diagram of Hypoeutectoid Steel 7-12
Table 11.1b Mechanical Characteristics of Hot-Rolled Material and Typical Applications for Various Plain Low-Carbon Steels 7-13
Applications - automobile body components. - structural shapes (I-beams, channel and angle iron) - sheets (used in pipelines, buildings, bridges, tin cans) 7-14
0.25 ~ 0.60 wt%c Medium-Carbon Steels May be heat treated by austenitizing, quenching, and then tempering to improve their mechanical properties Stronger than low-carbon steels and weaker than high-carbon steels Typical Tensile Properties for Oil-Quenched and Tempered Plain Carbon a Chapter Classified 7 as high-carbon steels 7-15
Applications - railway wheels and tracks - gears - crankshafts 7-16
High-Carbon Steels 0.60 ~ 1.4 wt%c Used in a hardened and tempered condition Hardest, strongest, and yet least ductile; especially wear resistant and capable of holding a sharp cutting edge Containing Cr, V, W, and Mo; these alloying elements combine with carbon to form very hard and wear-resistant carbide compounds (e.g., Cr 23 C 6, V 4 C 3, and WC) Applications: cutting tools and dies for forming and shaping materials, knives, razors, hacksaw blades, springs, and high-strength wire 7-17
Applications of High Carbon Steels Applications - cutting tools - drills - high-strength wires - spring materials - wires and Springs 7-18
Comparison of the Advantages Offered by Carbon Steels and Alloy Steels Carbon Steel Lower cost Greater availability Alloy Steel Higher strength Better wear Toughness Special high temperature behavior Better corrosion resistance Special electrical properties 94XX Ni- Alloy steel is more expensive than carbon steel; it Chapter should 7 be used only Copyright when 2007 a Dr. special Ali Ourdjini. property is needed. 7-19
Plain carbon steels are relatively cheap, but have a number of Property limitations. These include: i) Cannot be strengthened above about 690 MPa without loss of ductility and impact resistance. ii) iii) iv) Not very hardenable i.e. the depth of hardening is limited. Low corrosion and oxidation resistance. Must be quenched very rapidly to obtain a fully martensitic structure, leading to the possibility of quench distortion and cracking. v) Have poor impact resistance at low temperatures. 7-20
Effect of Alloying Elements Hardenability - Alloy steels have high hardenability. Effect on the Phase Stability - When alloying elements are added to steel, the binary Fe-Fe 3 C stability is affected and the phase diagram is altered. Shape of the TTT Diagram Increase strengthen, hardness and toughness of the steel Improve corrosion and wear resistance 7-21
Effect of Alloying Elements on TTT diagram 7-22
Alloying Elements Alloying elements combine in one of two ways: Form solid solution with ferrite: strengthen the ferrite Combine with carbon to form carbides: retard the softening rate, resulting in greater toughness 7-23
γ - stabilisers γ α - stabilisers 7-24
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