Investigating residual stresses in samples de-stressed by thermal and ultrasonic methods

Dissertation for receiving a master's degree in mechanical engineering

Manufacturing and production trend

Abstract

One ??of the important topics in the field of welding in engineering structures is the discussion of reducing residual stresses in welding lines and improving the quality of welding. Residual stresses reduce the load bearing limit, the life of welded joints and change the forms in welded structures. The purpose of this research is to investigate the residual stresses in samples de-stressed by two methods, thermal and ultrasonic. In this research, two sheets of carbon steel are divided into three equal parts after preparation and welding. Sample A was de-stressed by thermal method and sample B by using ultrasonic shocks. In the next step and after destressing two samples A and B, the central hole method was used to determine the residual stress of each sample. Sample C is another sample on which no stress relief operation has been performed and its residual stress value is also determined by the mentioned method. After that, in order to validate the results obtained from the central hole method, the samples were tested by X-ray diffraction method. The results showed that stress relief using ultrasonic shocks compared to thermal stress relief results are more favorable and the residual stresses in sample B are also lower.

Key words: stress relief with ultrasonic shocks, thermal stress relief, central hole method, X-ray diffraction standard.

) Introduction

Most of the structure in the industry, from various parts made by processes such as casting, forging, rolling, machining, welding, etc. They are produced, formed and connected to each other with the help of various processes. Various connection methods can be classified into the following groups based on practical and scientific processes:

Mechanical connections (screws, rivets, nails, pins, etc.)

Metalological connections (welding, soldering, etc.)

Chemical connections (organic and inorganic adhesives)

Each of these connections are placed in one of the following categories according to the tasks they are responsible for:

Temporary connections such as bolts and nuts, pins, etc.

Semi-temporary connections such as rivets, soft soldering and some adhesives

Permanent connections such as welding processes, solder Hard work and most adhesives with chemical reactions

Welding processes as processes to create molecular connection between two or more metal pieces, at least one of which has been localized melting or turned into a paste due to heat, for both the purpose of making primary parts and also their connection has a wide application in industrial societies. The economic efficiency of these connections compared to other connections in terms of longer life and the confidence it creates due to the use of materials similar to the main material are some of the reasons for the preference of this process [1] [.

Among the processes that are considered after welding, in other words, in the welded parts, are the residual stresses due to the successive heating and cooling of the weld and its nearby areas and the inability to move in some directions, internal residual stresses arise in the weld and its adjacent areas.

Residual stress is the stress that remains in the body as a result of performing specific operations such as welding, and it exists even when the body is not under any external loading]19[

Stress relief, which is also called PWHT or post-heat treatment, is a process, in order to Eliminating existing tensions that have arisen in the operation and production process. There are different methods to eliminate residual stresses in welded samples, among which we can mention the thermal method and the ultrasonic method. [16] [

There are different techniques for measuring the residual stress, which require special devices for measurement in each method.

1-2) Sectioning of the contents

According to the mentioned contents and the topic of the thesis under the title of investigating the residual stresses, in the samples welded by thermal and ultrasonic methods, the contents will be divided into several chapters, which will be explained in the following.

In this chapter, we will first mention the introduction of various stress relief methods. After that, we will explain the types of residual stress measurement methods.

The second chapter will be related to the research background and an overview of the research done, so that it will first refer to the studies done in the field of welding stress relief. The studies carried out in this section include two main parts, i.e. welding stress relief with ultrasonic blows and thermal stress relief, which will be explained in the relevant chapter. In the continuation of the second chapter, the studies carried out in the field of measuring residual stresses using the central hole method and the standard X-ray diffraction method will be mentioned.

The third chapter related to the research method includes the method used for welding the desired samples and the parameters considered during the welding process, introducing the stress relief device in two thermal and ultrasonic methods, how to prepare the samples for stress relief by thermal and ultrasonic methods, sample conditions and preparation It is to measure the residual stress using the central hole method and the standard X-ray diffraction method. The fourth chapter will examine the results obtained from the central hole method, the standard X-ray diffraction method, as well as the amount of residual stresses in each of the samples. Payment.

1-3) How to form residual stresses

As it was said, residual stress is the stress that remains in the object due to certain operations and it exists even when the object is not under any external load. The nature of residual stress is such that there must be a compressive stress in front of every tensile stress, so that the object remains in a state of equilibrium, which is called a self-equilibrium state. So, if we have tensile residual stress in a region and our loading is also tensile stress, the stress level in that region will be higher than what is obtained only in terms of external tensile stress. Therefore, if we do not consider the internal residual tensile stress and design the part only based on external applied stress, it is possible that due to the external residual stress, the stress in the part may exceed its yield limit and cause it to fail. The internal residual occurs in the weld and its adjacent areas.

The amount of expansion and deformation of the object against heat is proportional to the temperature. Basically, with the increase in temperature up to the melting point of the metal, we will see expansion in them. Now, when the temperature locally increases in a point of the object, a thermal gradient is created around it, which wants to cause the change in shape and expansion of the point whose temperature has increased, but the change in the shape of this point is countered by the metal that surrounds this point and does not want to change shape, so the areas near this point are subjected to compressive stress, and if the existing compressive stress exceeds the yield limit, it causes the plastic deformation of this area. will be During the cooling of the area that was heated, we will witness a local contraction that will cause tensile stress in the vicinity of this point to the limit of the yield stress of the metal.]19[

1-3-1) Description of residual stress in welding

If we want to explain the formation of residual stress in welding, the following model can be described:

Consider three bars one, two and three bounded on both sides by rigid plates four and five