第一章Weldabitliy and Testing

This chapter is mainly about the introduction of weldabiltiy and how to evaluate the weldabitly of metals and alloys.

●1.1Weldabitliy

Weldabitliy A material's weldability refer to more qualitative characteristics of how easy or difficult it is to achieve a functional weld. It is depend on a variety of factors and not easily to be quantified. In evaluating weldability we employ the concept of fitness for purpose, including the difficulty in obtaining a sound welded joint, necessity of using pre-heating control of the thermal regime of welding, limitation of the specific heat input etc.

●1.2Susceptability Testing

Cracking susceptibility testing is widely used to assess the fabricability of materials by various welding processes and to determine the service performance of welded construction. Only a few of these have been standardized. ISO 17641 and 17642 provide procedures for hot cracking and cold cracking tests, respectively. These standards include the Varestraint and PVR tests for solidification cracking and the controlled thermal severity (CTS) and Y-groove (Tekken) tests for HIC. A summary of the weldability tests that are covered by the ISO standards is provided in in this section.

第二章Carbon and Low-alloyed Steels

From a weldability standpoint, carbon and low-alloyed steels can be divided into six groups according to composition, strength, heat treatment, including carbon steels, HSLA steels, QT steels, HTLA steels, Cr-Mo steels, etc. Regardless of the product form, the composition, mechanical properties, and condition of heat-treatment need to be known to establish satisfactory welding procedures, and the effect of size or thickness with respect to heat input, cooling rate, and restraint also need to be taken into consideration. This chapter will discuss the weldability of carbon and low-alloyed steels above in detail.

●2.1Carbon steels

Carbon steels including low-carbon steels, medium-carbon steels, and high-carbon steels, are commonly engineering alloys. However, they are with different weldability due to carbon content. In this section, we classify the metals and alloys, especially, the steels. And then, we focus on welding concerns and some remedies of carbon steels, especially on solidification cracking, hydrogen induced cracking, and toughness decrease.

●2.2QTLA steels

In this section, we focus on the weldability of QTLA steels. We first introduce properties of QTLA steels, especially the chemical, mechanical properties. Then, heat treatments and microstructures of QTLA steels are discussed. We also discuss the phase transformation of QTLA steels during welding, and introduce auto-tempering, the characteristic of QTLA steels. We list fundamental factors to auto-tempering. And then, we turn to welding concerns of QTLA steels, especially liquation cracking, hydrogen induced cracking, and toughness decrease. Finally, we supply some remedies to these problems.

●2.3HTLA steels

In this section, we focus on the weldability of HTLA steels. compared with QTLA steels, the basic information of HTLA steels,including the chemical composition, heat treatments, microstructures and mechanical properties have been introduced. The effect of thermal cycle on phase transformation of HTLA steels during welding has been addressed, and then the process of auto-tempering as well. The main welding concerns and remedies of HTLA steels will be discussed in detail, especially solidification cracking, liquation cracking, hydrogen induced cracking, and embrittlement, etc.

●2.4Case study

There are 4 cases in this section. The 1st part is focus on the question “what can we do with weldability analysis in designing welding procedure”. And in this part, we analyze the weldability of WELDOX 960 steels. And according to this analysis, we pay attention to hydrogen induced cracking, and decide the preheat temperature then. In the end of this part, we estimate this design by welding tests. The other 3 cases focus on analyzing failure causes of socket/pipe weldment, oil engine connecting rod, and water turbine pressure pipeline, and providing suggestion of remedies.

第三章Stainless Steels

Stainless steels are widely used in various industries because of their resistance to corrosion. The welding of stainless steels, especially the austenitic grades, is important in energy-related systems. This chapter summarizes typical welding problems in stainless steels and some recommended solutions. The problems associated with the fusion zone and the partially-melted zone will be addressed. We will focus on the HAZ phenomena in different grades including austenitic grades, ferritic grades and duplex. At last, some concern about dissimilar welding will be addressed.

●3.1Introduction

This section provides fundamental knowledge of classification and application of different series of stainless steels based on their chemical composition and corrosion properties. The corrosion resistance mechanism is discussed according to the galvanic cell. The roles of elements, phase diagram and metallurgy of stainless steels are analyzed.

●3.2Austenitic stainless steels

Welding concern of austenitic steels including corrosion attack due to precipitation of chromium carbide at GB in both weld and fusion boundary are addressed in this section. Based on the weld decay, knife-line attack, stress corrosion cracking and hot cracking,the mechanism is discussed including what does it look like, how does it occur, and how to remedy.

●3.3Ferritic and Martnsitic grades

The features, metallurgy and properties of martensitic and ferritic stainless steels are introduced. Based on phase diagram and thernal cycle, the cause and remedy of weld decay in ferritic and under bead cracking martnsitic stainless steels are discussed,respectively.Filler matirials and welding procedure are suggested as well.

●3.4Duplex stainless steels

The features, metallurgy and properties of duplex stainless steel are diccussed.The welding concern of phase balance and depress the formatrion of sigma phaase are analyzed.

●3.5Dissimilar materials

Carbon plates with stainless steel cladding layer is attractive due to lwer cost and better combination of properties, however, the mutual diffusion between base metal and cladding metal affect the corrosion resistance of the joint during manufacturing. how is mutual diffusion going on across the interface and how does it affect the properties of the joint? how to use a buttering layer to depressed mutual diffusion? all these questions will be discovered in this section.

●3.6Case study

Two cases of failure inculidng a pipe and a large exhaust fan are discussed in this section. How to determin the type of failure based on fracture, position, working conditions, etc is adressed. Some suggestions are provided.

●3.7Problems

Several problems will be explained in detail in this section.

第四章Aluminium Alloys

Aluminum alloys are more frequently welded than any other types of nonferrous alloys because of their widespread applications and fairly good weldability. In general, higher strength aluminum alloys are more susceptible to hot cracking in the fusion zone and the PMZ and losses of strength/ ductility in the HAZ. This chapter summarizes typical problems in aluminum welding and recommended solutions. The problems associated with the fusion zone and the PMZ will be discussed. We will focus on the HAZ phenomena in heat-treatable aluminum alloys, which are strengthened through precipitation hardening including the 2000, 6000, and 7000 series

●4.1Introduction

This section provides fundamental knowledge of classification and application of different series of aluminum alloys based on their chemical composition, heat treatment or forming processes. The roles of alloy elements and heat treatment on microstructure and mechanical properties of aluminum alloys are addressed. How does heat treatment, especially age-hardening works. principles of age-hardening.Heat treatment usually involves three stages, to dissolve the alloying elements at relatively high temperature to get homogeneous solid solution. rapid cooling or quenching the solid solution to obtain supersaturated solid solution (SSSS) of these elements in aluminum, to form a finely dispersed precipitates by controlled decomposition of the SSSS at appropriate temperatures.

●4.2Welding Concern

Welding concern including hydrogen porosity, solidification cracking, liquation cracking are addressed in this section. Including what does it look like, how does it occur, and how to remedy.

●4.3Al–Cu–Mg alloys

The heat-treatable alloys are known to have a tendency to overage during welding no matter in the artificially or naturally aged conditions. The reversion of or GP zones suggests overaging and hence inability to recover strength by postweld artificial aging. The microstructure and properties of aluminum alloys that was welded in the T6 or T4 conditions will be discussed based on thermal cycle and participation curve. The effect of PWNA and PWAA heat treatment will be addressed as well.

●4.4Al-Zn-Mg alloys

Unlike 6000- or 2000-series aluminum alloys, Al–Zn–Mg alloys ages much more slowly, have a much smaller tendency to overage during welding, and recover strength slowly but rather significantly by natural aging. For these reasons, Al–Zn–Mg alloys are attractive when postweld heat treatment is not practical. The properties of Al–Zn–Mg alloys that was welded in the T4 and T6 conditions will be discussed. The effect of PWNA and PWAA heat treatment will be addressed as well.

●4.5Problems

Several problems will be explained in detail in this section.

第五章Cast Irons

Cast irons are iron based alloys containing more than 2% carbon, 1 to 3% silicon and up to 1% manganese. As cast irons are relatively inexpensive, very easily cast into complex shapes and readily machined, they are an important engineering and structural group of materials. Unfortunately not all grades are weldable and special precautions are normally required even with the so-called weldable grades. In this chapter, the types of cast irons and main properties will be introduced. Several cases will be provided to show how to repair welding cast irons with or without preheating.

●5.1Introduction

Cast irons can be conveniently grouped according to their microstructure and properties. grey cast iron is inherently brittle and often cannot withstand stresses. the graphite clusters in malleable irons, and the nodular graphite in SG irons, give significantly higher ductility. The weldability may be lessened by the formation of hard and brittle microstructures in the heat affected zone (HAZ). Because of the possibility of casting defects and their inherent brittle nature, repairs to cast iron components are frequently required. In this section, classification and weldability of cast irons will be addressed.

●5.2Case study

Two cases will be introduced in this section. How to braze a cast iron part in correct way to prevent cracking and hardening, how does metal stitching work as a type of cold mechanical process for repairing cast iron, and how does to carry out no preheat welding repair of cast irons will be introduced.