Welding Basics
The mechanical properties of metals include strength, ductility, hardness, toughness, fatigue resistance, among others.
Hardness can be measured using several methods, including Brinell hardness, Rockwell hardness, and Vickers hardness.
Thermal conductivity refers to the ability of metallic materials to conduct heat.
The physical properties of metals include density, melting point, thermal expansion, thermal conductivity, electrical conductivity and others.
Heat-resistant steel is steel that maintains a certain thermal stability and strength at high temperatures.
Stainless steel is a type of steel that is resistant to corrosion from substances such as air, acid, alkali and salt, thanks to its stainless and acid-resistant surface.
The ability of metallic materials to conduct electrical current is called electrical conductivity and is measured by resistivity.
The temperature at which a metal changes from a solid to a liquid state is called its melting point.
Weld symbols generally consist of basic symbols and leaders, with additional symbols such as auxiliary symbols, supplementary symbols, and weld size symbols added as needed.
The symbol for a level weld surface is represented by a dash (-), while a symbol representing a backer plate at the bottom of the weld is represented as a circle with a cross inside (). A symbol for a weld around the weldment is represented by a circle (○).
Heat treatment can be classified into annealing, quenching, normalizing and tempering based on the methods used for heating and cooling.
A direct current (DC) is a type of electrical current whose direction and magnitude remain constant over time, while an alternating current (AC) fluctuates.
Lamellar tear is a type of stepped crack that forms along the lamination layers of a steel plate in welded components due to welding.
When other conditions are constant, the tendency for porosity to occur increases with increasing welding speed. Likewise, an increase in welding current and arc voltage will lead to an increase in the tendency to porosity.
Manual arc welding is a versatile method that can be used to weld various materials such as carbon steel, low alloy steel, heat resistant steel, low temperature steel and stainless steel.
Manual arc welding can be performed in various positions, such as horizontal, vertical and oblique, making it a widely used method.
Compared to gas welding and submerged arc welding, manual arc welding results in finer microstructure, smaller heat-affected zones, and better joint performance.
The quality of the final product during manual arc welding is directly influenced by the technical proficiency and experience of the welder.
Welding process parameters refer to various physical quantities selected to ensure the quality of the welding process.
The choice of electrode for manual arc welding is mainly based on the properties of the base metal, the type of joint and the working conditions. The resistance grade of the electrode for welding general carbon steel and low-alloy steel is mainly selected according to the principle of equal resistance.
Many factors must be considered when selecting welding current for manual arc welding, but the main factors include electrode diameter, welding position, and weld bead level.
The empirical formula for calculating welding current for manual arc welding is I = 10d^2.
The basic groove shapes commonly used in manual arc welding include I-shape, V-shape, double V-shape, double Y-shape, and double U-slot with bell edge.
Welding cracks have the characteristics of a sharp notch and a large aspect ratio.
The purpose of preheating is to slow the cooling rate and improve stress conditions.
The slag produced by an alkaline electrode has strong sulfur and phosphorus removal ability.
Immediately after welding, all or part of the welding must be subjected to heating or insulation. The slow cooling process is called post-heating and is crucial in preventing delayed cracking by effectively reducing hydrogen in the welded joint.
During welding, if bubbles in the pool of molten material do not escape during solidification, the resulting holes are called porosity.
The residual slag dissolved in the weld after welding is called slag inclusion.
The coating layer applied to the surface of the welding core is called cladding.
The length of the welding rod depends on factors such as the diameter of the welding core, the material and the type of coating.
Steel for welding core can be divided into three categories: carbon structural steel, alloy structural steel and stainless steel.
Welding rods can be divided into different types depending on their purpose: carbon steel electrodes, low alloy steel electrodes, stainless steel electrodes, cladding electrodes, cast iron electrodes, nickel and nickel alloy electrodes, electrodes copper and copper alloy, aluminum and aluminum alloy electrodes and special purpose electrodes.
Related Reading: How to Choose the Right Welding Rod?
A slotless butt joint is used for welding thinner steel plates, while a slotted butt joint is used for full penetration welding on thicker steel plates.
An acetylene generator is a device that generates acetylene through the interaction of voltage and water.
The purpose of a pressure reducing valve is to reduce the high pressure gas stored in the gas cylinder to a stable working pressure.
The mixing ratio of oxygen and acetylene, known as β, can result in a neutral flame with a β value between 1.1-1.2, an oxidation flame with β greater than 1.2, or a carbonization flame with β less than 1.1.
The parameters of a gas welding process include welding wire diameter, flame properties and energy efficiency, welding nozzle bias and welding speed.
Welding is a processing method that allows two metal parts to be joined by heating, pressure or both, with or without the use of filler materials, resulting in an atomic bond.
There are 13 basic symbols that represent the cross-sectional shape of a weld.
The angle between the vertical of the welding surface and the groove surface is called the groove surface angle.
The junction between the weld surface and the base metal is called the weld toe.
When welding non-ferrous metals, cast iron, stainless steel and other materials, gas welding flux is typically required.
Related Reading: Ferrous vs Non-Ferrous Metals
The safety of an acetylene cylinder is guaranteed by a fusible plug located on the cylinder shoulder.
Methods for lighting a welding arc include the momentary contact method and the high-frequency, high-voltage arc method.
The standard structure of manual arc welding is the acid electrode, while important structures require the use of an alkaline electrode.
Electrodes can be divided into acidic electrodes and alkaline electrodes based on the slag-dissolving properties of their coatings after melting.
The safety of an oxygen cylinder is guaranteed by a metal safety disc on the cylinder valve.
The metal cutting process involves three steps: preheating, heat burning and slag removal.
In heat treatment production, common cooling methods include isothermal cooling and two types of continuous cooling.
The generation and maintenance of an arc requires cathode electron emission and gas ionization.
The main sources of sulfur in a weld are the base metal, welding wire, flux coating or flux, therefore controlling the sulfur content in the welding raw materials is crucial to reducing the sulfur content in the weld.
In fusion welding, the amount of energy supplied per unit length of weld is called linear energy.
Primary molten pool crystallization involves two processes: nucleation and nucleation growth.
When welding with CO2 gas shielding, nitrogen porosity may occur due to insufficient shielding or impure CO2 gas.
The reverse wire mode of CO2 gas shielded semi-automatic welding includes push wire type, drawn type and push-pull type. Currently, the push wire welding gun is the most commonly used.
CO2 gas shielded welding gas supply system consists of gas cylinders, dryer, preheater, pressure reducer, flow meter and other components.
CO2 gas shielded welding equipment includes a welding power source, welding torch, wire feeding system, air supply unit and control system.
Post-weld heat treatment may include post-weld heat treatment, high-temperature tempering, normalizing, or normalizing plus tempering.
Cracks that occur during welding of gray cast iron can be thermal stress cracks or hot cracks, with thermal stress cracks being the most common.
For hot welding of nodular cast iron, Cast 238 electrode must be used.
The stress and strain of welding in the direction perpendicular to the weld are called transverse stress and strain.
In structures with asymmetrical welds, the side with the fewest welds should be welded first and then the side with the most welds to minimize overall deformation.
To counteract welding deformation, the welding can be artificially deformed in the opposite direction before welding. This method is known as the reverse deformation method.
The heat dissipation method is not suitable for materials with high hardenability.
The flame correction method involves using the plastic deformation generated by local flame heating to compress the metal after cooling in order to correct the deformation.
The key to successful flame heating correction is to understand the deformation caused by local flame heating, determine the correct heating position, and control the temperature and number of repeated heatings.
Flame heating modes include spot heating, linear heating and triangular heating.
Triangular heating is often used to correct bending deformation in components with thick and strong stiffness.
Preheating before welding can reduce temperature differences and delay cooling, thereby reducing welding stress.
An acetylene cylinder must be used vertically and, if placed horizontally, must rest for 20 minutes before use.
The pressure reducer must not be contaminated with grease during use.
Before using the welding torch, it is important to check its injection and absorption performance.
When welding stainless steel plates, three different Dacron electrodes must be used for the same weld.
If a welding torch backfires, turn off the acetylene first and then the oxygen.
According to the provisions of the national standard GB3323-82 “Classification method for steel weld radiography and negatives”, the quality standard for steel weld radiography is divided into four levels, with fewer defects in Grade I and more in Grade IV welds. .
GB3323 states that cracks are not permitted in Grade I welds and there must be no lack of fusion, incomplete penetration, inclusion of strip slag or cracks. For Grade II welds, there shall be no bilateral welding or incomplete penetration in one-sided welding with backer plate.
When using a carbon steel electrode to connect a joint of cast iron and low carbon steel, a 4-5mm insulation layer can be deposited on the cast iron groove first, and then mounting spot welding can be carried out after cooling.
