Metallurgy and Technology of Steel Castings

Jan Głownia

• Publisher: Bentham Science Publishers
• Year: 2017
• ISBN:
  9781681085708
• Topic: Tecnologia e ingegneria
• Subtopic: Ingegneria mineraria

Metallurgy and Technology of Steel Castings is a comprehensive textbook for students and professional engineers in the field of foundry engineering and materials science. The topics covered in this book explain the association between the quality of liquid metal and the properties of the finished cast. Readers will learn about the thermodynamic conditions for addition and recovery of chemical elements (such as Cr, Ni and Mo) in steel, degasifying processes, the influence of alloying additives for manufacturing steel castings that operate in extreme temperatures, anti-corrosive steels and basic cast conditions for making the castings (pouring and heat treatment systems).
Metallurgy and Technology of Steel Castings gives readers essential information about steel and steel cast manufacturing processes and equips them with the knowledge to overcome the challenges faced in the foundry environment.

Cast Steels for Service Conditions

Jan Głownia

Abstract

Low-alloy cast steels are used in many branches of industry for various mechanical parts with very different wall thicknesses (40-400mm). The mechanical properties in such sections must be equal throughout. For these reasons, there are several grades of cast steels, which fulfil the requirements for different uses. This chapter characterises the parameters of low-alloy cast steels, intended for several service conditions.

Keywords: Cr-Ni-Mo, Effect of Cr –Ni, Low-alloy cast steel, Mechanical properties, Wall thickness.

1. Introduction

Cast steels of this group are the basis of all machine constructions due to their optimum strength properties compared to their plastic properties. The application areas of the castings made of these types of cast steel are primarily in the following industries: the machinery manufacturing industry; the automotive industry - heavy vehicles; the marine industry - ships, drilling platforms; extractive industries - excavators, bulldozers, mining crushers; and the energy industry - turbines. Castings designed for such working conditions have a wall thickness from 50 to 400 mm and are subject to strong dynamic loads. Very often, they work under varying climatic conditions, over large temperature differences, from + 20 to minus 60°C. Such items include wagon wheels, axle boxes and other parts of wagons and electric locomotives. Therefore, a high yield strength, a high value of YS/UTS, impact toughness and good weldability are required from these castings, as they are very often welded with rolled or forged components.

The use of carbon steel for castings of this type, with wall thicknesses above 30 mm, is limited, because without the introduction of alloying elements, a homogeneous structure and properties throughout the cross section of the casting walls cannot be obtained.

The limited application of carbon cast steel is also connected with the difficulty in obtaining a tensile strength above 650-700 MPa. Raising the carbon content or the

application of more complex heat treatments to obtain a UTS above 700 MPa can lead to elongation and too low an impact toughness (Fig. 1). Thus, in these cases where a tensile strength of 700-1200 MPa is required in castings with wall thicknesses from 30 to 100 mm, alloying elements should be introduced into the cast steel. Therefore, cast steels containing silicon, manganese, chromium, nickel or molybdenum in particular are often found in use. The added elements’ influence on the mechanical properties of cast steel should be considered comprehensively, because there are usually walls of various thicknesses in castings, and the properties and structure must be equal throughout the casting. However, due to unequal cooling rates in walls of different thickness or wall connections, each of them has a different structure, and often separation of carbides, sulphides or oxy-sulphides also occurs. Therefore, elements reducing the critical quenching rate are introduced into cast steel. These allow homogeneous structure and properties throughout the entire wall thickness of the casting to be achieved. These features are usually obtained after the introduction of not one, but several, alloying elements. The synergistic effect of elements such as Si, Cr, Mn, Mo and Ni means that cast steels, such as Mn-Si, Cr-Mo, Cr-Ni-Mo or Cr-Ni- Mo-V (Table 1), are often used in practice.

Fig. (1))
Mechanical properties of carbon cast steel [1].

Table 1 Cast steel after quenching and tempering, intended for thick-walled castings.

ASTM		DIN	A27 60-30 62 --35 GS-Ck25						GS-30 Mn 5							GS-25 Cr Mo 4									GS–34 Cr Mo 4											GS–42Cr Mo 4												GS–34 Cr Mo 44												GS–30Cr Mo V 64												G–35 Cr Mo V 104												GS–40 Ni Cr Mo 656
Quality class				I		II				I			II				I			II			III			I				II				III			I				II				III				I				II				III				I				II				III				I				II				III				I				II				III
Wall thickness, mm				Mechanical properties
up to 30		Re, MPa		245		275				345			440				390			510			635				510				635			765				540				655				835				510				635				765			540			685				835			540					685				835			540					685				885
		Rm, MPa		440- 590		490- 640				590- 740			740- 880				590-- 740			740-880			880-1030				740-880				880-1030			1030- 1180				740-880				880-1030				1030- 1180				740-880				880-1030				1030- 1180			740-880			880-1030				1030 1180			740-880...