The characteristics of metallic materials are the main basis for material selection. Metal material performance is usually divided into process performance and service performance. Performance refers to the performance of metallic materials under the conditions of use of metallic parts. The characteristics of metallic materials determine the scope of their use. Performance includes physical properties, chemical properties and mechanical properties.
01 Physical properties
The characteristics that metals reflect when subjected to physical influences such as force, heat, light, and electricity are the physical properties of metals. The main physical indicators are shown in Table 1.
Table 1. Physical properties of metals
The chemical properties of metallic materials refer to the ability of metallic materials to resist chemical erosion when exposed to various corrosive environments at room or high temperature. The chemical properties of metallic materials mainly lie in their corrosion resistance. The ability of metallic materials to resist corrosive environmental damage is called corrosion resistance.
Chemical corrosion is the result of direct chemical interaction between a metal and its environment, which includes two forms of corrosion: gas corrosion and metal corrosion in a non-electrolyte. Its feature is that no current is generated during the corrosion process, and corrosion products are deposited on the metal surface. For example, the rust phenomenon caused by pure iron in water or steam and gas at high temperature is a typical example of chemical corrosion.
Corrosion caused by the action of metals in contact with electrolyte solutions such as acids, alkalis and salts is called electrochemical corrosion. It is characterized by the formation of current in the process of corrosion (the so-called micro-accumulator effect), and its corrosion product (rust) does not cover the metal surface, like an anode, but at a certain distance from the anode metal. It is generally believed that the cause of electrochemical corrosion is related to the electrode potential of the metal. The process of electrochemical corrosion is much more complicated than chemical corrosion, and its danger is also relatively high. Metallic materials are susceptible to corrosion damage, mainly related to this type of corrosion.
Table 2. Common types of metal corrosion
Corrosion rate refers to the overall rate of corrosion (i.e., uniform corrosion) of a material, obtained by placing a specimen in a test environment and measuring the change in its weight over a period of time. The corrosion rate can be expressed in terms of mass loss per unit time and per unit area, and the calculation formula is as follows:
Table 3. Classification and level of corrosion resistance of metallic materials
03 mechanical properties
The mechanical properties of a material refer to the performance of a material when subjected to various external loads (tensile, compression, bending, torsion, impact, alternating stress, etc.) under various conditions (such as temperature, environment, humidity). ) mechanical characteristics. Due to different ways of applying loads, as well as very complex environmental and environmental changes, the behavior of metals under these conditions will be very different, which leads to a wide range of studies of the mechanical properties of metallic materials. Developed, it became a marginal subject between metallology and mechanics of materials. Since the load-bearing conditions of metal components are usually expressed in terms of various mechanical parameters (such as stress, strain, impact energy, etc.), people refer to the critical value or setpoint as mechanical parameters that characterize the mechanical behavior of the metal. materials as the mechanical properties of metal materials. Indicators such as strength index, ductility index and toughness index, etc. The mechanical properties of metals are shown in Table 4.
Table 4. Mechanical properties of metals
04 welding performance
Metal weldability is the ability of the metal material itself to adapt to the welding process. It mainly refers to whether high-quality welds can be obtained under certain welding process conditions (including welding consumables, welding methods, welding process parameters, and structural shapes). etc.) The degree of sophistication and the ability of the connection to operate reliably under specified conditions of use. It includes two aspects: one is the joint performance of welded joints, that is, the possibility of obtaining high-quality and defect-free welded joints under certain conditions of the welding process; compliance with various operating conditions provided for by the technical requirements. There are many factors that affect weldability. For iron and steel materials, there are factors such as selected materials, joint structure and design, process methods and specifications, and environmental conditions for joint operation.
Basic structure of the heat-affected zone of a welded joint
Welded joints typically include a weld metal zone, a fusion line, and a heat affected zone. The heat-affected zone is the area in which the structure and properties of the metal on both sides of the weld are changed due to welding heating. Changes in the microstructure and properties of the heat-affected zone depend not only on the resulting thermal cycle, but also on the composition and initial state of the base metal, as shown in Figure 2.
Figure 2. Characteristics of the distribution of the heat-affected zone of welding 1-fusion zone, 2-overheating zone,3-zone of normalization, 4-zone of incomplete recrystallization, 5-base metal, 6-hardening zone, 8-tempering zone< /p>
Distribution of microstructure and properties of heat-affected zone of hardened steel
Hard to harden steel refers to a steel that is difficult to form martensite under natural cooling conditions after welding, such as ordinary mild steel. As shown in fig. 2, the heat-affected zone of difficult-to-harden steel consists of four parts: a melting zone, an overheating zone, a normalization zone, and an incomplete crystallization zone.
(1) Fusion zone. The melting zone includes the melting zone and the filler metal semi-melting zone (i.e., the heating temperature is between the liquidus line and the solidus line), and the semi-melting zone has low strength and toughness due to large heterogeneity in chemical composition and organizational properties., should be paid Attention.
(2) Hot zone. The heating temperature is usually around 1100°C, and the grains in this region begin to grow rapidly, and after cooling, a coarse superheated structure is obtained, also called the coarse grain region. This area is prone to brittleness and cracking.
(3) Normalization zone (phase recrystallization zone). When the heating temperature is higher than Ac3 and the grains begin to grow rapidly, the grains in this region do not grow significantly, and after cooling, pearlite and ferrite are uniform and fine, which is equivalent to the normalizing structure of heat treatment and has a good complex performance.
(4) Incomplete recrystallization zone. The heating temperature is within Ac1~Ac3, the structure in this area is uneven, the grain size is different, and the mechanical properties are not the same.
The above four zones are the main organizational characteristics of the low carbon and low alloy steel heat affected zone. However, some base metals are cold rolled or cold worked before welding, and the metal is subjected to a recrystallization process in a heating temperature range close to 500°C-Ac1, so that the work hardening effect disappears, the strength is reduced, and the ductility is improved. However, for aging-sensitive steels, if the time in the temperature range of Ac1-300°C is somewhat longer, then strain aging easily occurs, making this area brittle, so this area is also called the aging embrittlement zone, although its metal structure. There are no obvious changes, but there is sensitivity to notches, so you should pay attention to this when welding.
Distribution of the microstructure and properties of the heat-affected zone of light-hardening steel
Easy hardenable steel refers to steel types in which martensitic and other hardened structures are easily formed by quenching under air-cooling conditions after welding, such as quenched and tempered steel and medium carbon steel.
(1) Full hardened zone. Heating temperatureis located between the solidus line and A, and due to the growth of grains in this region, coarse-grained martensite is obtained, at a different cooling rate, a mixed structure of martensite and bainite may also appear. The hardened structure is prone to brittleness and cracking.
(2) Incomplete hardening zone. The heating temperature is within Ac1-Ac3, which corresponds to the zone of incomplete recrystallization. Depending on the content of elements in the base metal or the rate of cooling, mixed structures such as bainite, sorbitol and perlite may also appear.
(3) Hardening zone. If the base metal is quenched and tempered before welding, there will be a tempering softening zone. If the quenching and tempering temperature of the base metal before welding is t1, then in the welding process, when the heating temperature exceeds this tempering temperature t1 (and less than Ac1), then the excessive tempering softening phenomenon . If it is below t1, the fabric efficiency remains unchanged.
Cracks in welds can be detected with the naked eye or flaw detection. Classification of welding cracks: for example, according to the location of the cracks, they can be divided into weld cracks, fusion cracks, root cracks, weld toe cracks, arc crater cracks, etc.; for example, according to the cracking mechanism, it can be divided into hot cracks, reheat cracks and cold cracks, stress corrosion cracking, etc. Welding cracks are the most serious defects in welded joints and are not allowed in structural and equipment members.