Designing Weldments. Группа авторов

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1

      Properties and Strength of Material

       Synopsis

      This chapter defines what is meant by the term weldment. How it is distinguished from other structures. What are the factors that should be considered in deciding the conversion of a Cast structure to a welded structure? This introduction chapter gives essential basic knowledge quired for design of weldments.

       Keywords

      Weldment, static, dynamic, structure, cast, forged, NDE, distortion

1.1 Introduction

      A weldment is an assembly of various members of a structure that are joined by welding. The word structure may include from a simple joint of two members, for a trailer hitch, motor vehicle chassis, building frame, or a more complex joint of say a bridge-spurs of road over pass, construction of a ship, of an offshore platform, where multiple members are joined by welding in complex configurations to address varying static or dynamic stresses. The joining of different members by welding to create a complex structure is a preplanned activity with following basic objectives.

      Of course, the technical feasibility, ease of fabrication, availability of the material, and the cost are the other important factors that are associated and woven with the basic objectives, and all those are considered at the design stage.

      Often to reduce the weight and/or the cost of a cast structures it may be selected for conversion to a welded structure. It may be noted with caution that a machines or a component constructed (Cast) out of a cast-steel or cast iron is not an automatic fit-design for welded design of the same component. This is a very common error among the designers of machine parts. Same caution also applies for any forged part, considered for the conversion to a welded design.

      Figure 1.1 A typical strain and stress diagram, describing various elements of tensile test.

      When welded design is used to convert Cast and Forged components, it can significantly reduce the cost of the component, the weight of the component, and also improve its aesthetics. But it needs very careful, and in depth study of both the designs, for usability, and the end objective of the component.

      As a minimum the weldment designer should have good knowledge and experience of the following manufacturing processes.

      The strength of steel is an important aspect of design, typically the tensile strength is used for the design purpose. A typical stress and strain diagram of steel shown the Figure 1.1 above with various strength related elements of the steel that can be determined by testing a specimen of the steel.

2

      Properties of Metals

       Synopsis

      This chapter discusses the properties of material, structure sensitive and structure insensitive properties are defined. How properties are determined for engineering applications. Behavior of metal in extreme environ conditions like heat and cold are introduced.

       Keywords

      Mechanical, physical, corrosion, modulus of elasticity, tensile strength, fatigue strength, cyclic loading, HAZ

2.1 Material Properties

      Knowledge of the properties of the metal is an essential aspect of welding engineer’s ability to be a good welding engineer. This knowledge allows the engineer to choose the most suitable material to improve upon the cost and functioning of the component being designed.

      Various metals and non-metals are used in fabrication and construction, they all possess certain specific properties that differentiates them from others to be more desirable for the specific demands of the design to be an engineering material. All these metal properties are assessed and classed in three specific metal properties that are relevant to the engineering evaluations, for the suitability for the project.

      Metal properties can be classified as,

      These are the primary properties however they can also be classified on the basis of their nuclear and optic properties. Further they can be classified on the basis of, if these properties are structure sensitive, or structure insensitive etc. Some details of these properties given the Table 2.1 below.

      Table 2.1 Properties of materials.