The purpose of the work: to study the structure of white, gray, malleable and high-strength cast irons, to get acquainted with their basic properties, marking, the dependence of the properties of cast irons on their structure.
Figure 1Fe–C alloy system diagram
Figure 2 Classification of cast iron by structure
We will describe separately each type of cast iron. As can be seen from Figure No. 2, there are only 9 varieties of cast irons according to the metal base and the shape of graphite inclusions.
Gray cast iron is marked with the letters SCH and numbers characterizing the magnitude of the tensile strength during tensile tests. Grades, mechanical properties and approximate composition of gray cast irons are given in Table.
As the graphite inclusions round off, they negative role as the undercutting of the metal base decreases, and the mechanical properties of the cast iron increase. The rounded shape of graphite is achieved by modification. Cast iron modifiers are SiCa, FeSi, Al, Mg. When magnesium is used as a modifier in an amount of up to 0.5%, introduced before casting, ductile iron with spherical graphite inclusions is obtained.
Mechanical properties and composition (%) of gray cast irons
according to GOST 14120–85
| Cast iron grade | σ in, MPa, not less than | Hardness HB, no more | Composition, %, no more | ||||
| FROM | Si | Mn | P | S | |||
| mid 10 | 3,5–3,7 | 2,2–2,6 | 0,5–0,8 | 0,3 | 0,15 | ||
| mid 15 | 3,5–3,7 | 2,0–2,4 | 0,5–0,8 | 0,2 | 0,15 | ||
| mid 20 | 3,3–3,5 | 1,4–2,4 | 0,7–1,0 | 0,2 | 0,15 | ||
| mid 25 | 3,2–3,4 | 1,4–2,4 | 0,7–1,0 | 0,2 | 0,15 | ||
| mid 30 | 3,0–3,2 | 1,3–1,9 | 0,7–1,0 | 0,2 | 0,12 | ||
| MF 35 | 2,9–3,0 | 1,2–1,5 | 0,7–1,1 | 0,2 | 0,12 |
Ductile iron is marked with the letters HF and a number characterizing the magnitude of the temporary resistance, for example HF 35. The mechanical properties of some ductile irons are given in Table. Ductile irons are used to make critical parts: gears, crankshafts.
Minimum mechanical properties and hardness
ductile iron according to GOST 7293–85
| Cast iron grade | σ in | σ 0.2 | δ | Hardness HB | |
| MPa | % | ||||
| HF 35 | 140–170 | ||||
| HF 40 | 140–202 | ||||
| HF 45 | 140–225 | ||||
| HF 50 | 153–145 | ||||
| HF 60 | 192–277 | ||||
| HF 70 | 228–302 | ||||
| HF 80 | 248–351 | ||||
| HF 100 | 270–360 | ||||
Ductile iron is marked with the letters KCh and the numbers of tensile strength and relative elongation, for example KCh 35–10. In table. 3 shows grades, mechanical properties and chemical composition some ductile irons. Ductile iron castings are used for parts operating under shock and vibration loads (crankcases, gearboxes, flanges, couplings).
Mechanical Properties and Chemical Composition (%) of Ductile Cast Irons
according to GOST 1215–79
WELDING GENERATORS AND CONVERTERS
DC power sources are divided into two main groups: rotary type welding converters (welding generators) and welding rectifiers ( welding rectifiers).
DC generators are subdivided: by the number of fed posts - into single-station and multi-station, by installation method - into stationary and mobile, by type of drive - into generators with electric drive and internal combustion engines, by design - into single-case and double-case. According to the form of external characteristics, welding generators can be with falling, rigid, gently dipping characteristics and combined type.
The most widespread are generators with falling external characteristics, operating according to the following three main schemes:
with independent excitation and demagnetizing series winding;
with magnetizing parallel and demagnetizing series excitation windings;
with split poles.
None of the three types of generators with falling external characteristics is distinguished by significant advantages both in terms of technology, and in terms of energy and weight indicators.
The welding converter contains a three-phase drive motor, a DC welding electric generator and a welding current control device.
The welding unit contains a drive internal combustion engine, a DC welding electric generator and a welding current control device.
Welding generators are divided by design into collector and valve, and by the principle of operation into generators with self-excitation and with independent excitation.
Collector-type welding generators with independent excitation were used in welding converters, the production of which was discontinued in our country in the 90s of the 20th century, but are still in operation in some organizations.
Other types of generators are currently integral part welding units.
Collector welding generators
Collector generators are DC machines containing a stator with magnetic poles and windings, as well as a rotor with windings, the ends of which are brought to the collector plates.
When the rotor rotates, the turns of its windings cross the magnetic field lines and an EMF is induced in them.
Graphite brushes make moving contact with the collector plates. The brushes of the machine are located on the electric (geometric) neutral of the collector, where the EMF in the turns changes its direction. If you move the brushes from the neutral, then the voltage of the generator will decrease and the switching of the windings will occur under voltage, which in welding generators under load will lead to a very rapid melting of the collector by an electric arc.
The EMF on the brushes of the welding generator is proportional to the magnetic flux created by the magnetic poles E2 = cF, where F is the magnetic flux; c is the constant of the generator, determined by its design and depending on the number of pairs of poles, the number of turns in the armature winding, and the speed of rotation of the armature.
Generator output voltage at load U2 = E2 - JsvRg, where U2 is the output voltage at the generator terminals at load; Jsv - welding current; Rg is the total resistance of the armature circuit section inside the generator and brush contacts.
Therefore, the external static characteristic of such a generator is gently falling. To obtain a steeply falling external static characteristic in collector generators, the principle of internal demagnetization of the machine is used, which is provided by the demagnetization stator winding. If it is necessary to obtain a rigid external static characteristic, a magnetizing stator winding is used.
Classification of welding converters and units. For DC welding, welding converters and welding units serve as power sources. The welding converter consists of a DC generator and a drive motor, the welding unit consists of a generator and an internal combustion engine. Welding units are used for work in the field and in cases where the voltage fluctuates greatly in the supply electrical network. The generator and internal combustion engine (gasoline or diesel) are mounted on a common frame without wheels, on rollers, wheels, in a car body and on a tractor base.
To work in different conditions the following units are produced: ASB-300-7 - a GAZ-320 gasoline engine mounted with a GSO-300-5 generator on a frame without wheels; ASD-3-1 - diesel engine and generator SGP-3-VIII - in the same version; ASDP-500 - like the previous unit, but mounted on a two-axle trailer; SDU-2 - unit mounted on the basis of the T-100M tractor; PAS-400-VIII - ZIL-164 type engine. and SGP-3-VI generator mounted on a rigid frame equipped with rollers for moving on a flat floor. Other units are also produced, differing in design.
Welding generators are single-station and multi-station, designed to simultaneously power several welding stations. Single-station welding generators are manufactured with falling or hard external characteristics.
Most of the generators that make up welding units and converters (PS and PSO types) have a falling external characteristic. The generator of the PSG type converter has a rigid current-voltage characteristic. Universal generators are produced, which make it possible to obtain both falling and hard characteristics (converters of the PSU type).
Welding converters PSO-500, PSO-ZOOA, PSO-120, PSO-800, PS-1000, ASO-2000, PSM-1000-4 and others are supplied mainly with asynchronous three-phase squirrel-cage motors in a single-case version. They have wheels to move around the shop or are mounted motionless on the plate.
Technical data of some converters are given in table. 51.
The device and operation of welding generators. The industry produces welding generators of three types: with independent and parallel excitation windings, demagnetizing series winding and with split poles.
Generators with an independent excitation winding and a demagnetizing series winding (Fig. 119) are mainly used in welding converters PS0420, PSO-ZOOA, PSO-500, PSO-800, PS-1000, ASO-2000, which differ in power and design.
On the generator diagram (Fig. 199, a) two excitation windings are shown: independent H and consistent FROM which are located at different poles. A rheostat is included in the independent winding circuit RT. The series winding is made of a large section bus, as a large welding current flows in it. From part of its turns, a solder was made, placed on the switch P.
The magnetic flux of the series winding is directed towards the magnetic flux created by the independent excitation winding. As a result of the action of these streams, the resulting stream appears. When idling, the series winding does not work.
The open circuit voltage of the generator is determined by the current in the field winding. This voltage can be adjusted with a rheostat. RT, changing the magnitude of the current in the circuit of the magnetizing winding.
When loaded, a welding current appears in the series winding, creating a magnetic flux in the opposite direction. As the welding current increases, the opposing magnetic flux increases and the operating voltage decreases. Thus, a falling external characteristic of the generator is formed (Fig. 119, b).
Change external characteristics regulation of the current in the independent excitation winding and switching the number of turns of the demagnetizing winding.
In the event of a short circuit, the current strength increases so much that the demagnetizing flux increases sharply. The resulting flux, and consequently the voltage at the generator terminals, practically drops to zero.
The welding current is regulated in two ways: by switching the number of turns of the demagnetizing winding (two ranges) and by a rheostat in the independent winding circuit (smooth regulation). When connecting the welding wire to the left terminal (Fig. 119, a) small currents are set, on the right - large currents.
Generators with parallel magnetizing and series demagnetizing excitation windings belong to the system of generators with self-excitation (Fig. 120). Therefore, their poles are made of ferromagnetic steel, which has residual magnetism.
As can be seen from the diagram (Fig. 120, a), the generator has two windings on the main poles: a magnetizing H and a demagnetizing C connected in series. The current of the magnetizing winding is created by the armature of the generator itself, for which the third brush is used FROM located on the collector in the middle between the main brushes a and b.
The counter-inclusion of the windings creates a falling external characteristic of the generator (Fig. 120, b). The welding current is smoothly regulated by the RP rheostat included in the self-excitation winding circuit. For stepwise current regulation, the demagnetizing winding is sectioned in the same way as in the PSO type generator. According to this scheme, generators of welding converters PS-300, PSO-ZOOM, PS-3004, PSO-300 PS-500, SAM-400 work.
The split-pole generator (Fig. 121) does not have a series winding. In this generator, the arrangement of the poles differs from conventional DC electrical generators. The magnetic poles do not alternate (the north is followed by the south, then again the north, etc.), but the poles of the same name are located side by side (two north and two south, Fig. 121, b). The horizontal poles Nr are called principal, and the vertical N p - transverse.
Rice. 121. Generator with split poles: a, b - basic magnetic and electrical circuits; F g i, F p i - magnetic fluxes of the armature, Fg - main magnetic flux, F p - transverse magnetic flux, GN - neutral, P - winding of the transverse poles, Gl - winding of the main poles, RT - rheostat
The main poles have cutouts that reduce their cross-section for full saturation of the magnetic flux already at idle. The transverse poles have a large cross section and operate in all modes with incomplete saturation. Only the main excitation windings are placed on the main poles, and only transverse ones on the transverse ones. An adjusting rheostat is installed in the circuit of the transverse excitation windings RT. Both windings are connected in parallel with each other and are powered by brushes, i.e. the generator operates with self-excitation. The generator has two main brushes a and b and extra brush With.
When loaded, a current appears in the armature winding, which creates a magnetic flux of the armature, magnetizing the main poles and demagnetizing the transverse ones. Since the main poles are completely saturated, the action of the magnetizing flux is not affected. With an increase in the welding current, the magnetic flux of the armature increases, its demagnetizing effect (against the flow of the transverse poles) increases and this leads to a decrease in the operating voltage; a falling external characteristic of the generator is created. Thus, the falling characteristic of the generator is obtained due to the demagnetizing effect of the armature magnetic flux.
Smooth regulation of the welding current is carried out by a rheostat in the circuit of the transverse excitation winding 1.
1 (In previously produced generators of this type (SUG-2a, SUG-26, etc.), coarse adjustment of the current was carried out by shifting the brushes from the neutral.)
According to the scheme with split poles, generators of converters PS-300M, SUG-2ru, etc. work.
Designs of single-station welding converters. Converters PS-300-1 and PSO-300 are used to power one post, for welding, surfacing and cutting. The converters are designed for operating current from 65 to 340 A.
The welding generator of the converter belongs to the type of generator with parallel magnetizing and series demagnetizing excitation windings.
The generator has steeply falling external characteristics (Fig. 120, b) and two ranges of welding currents: 65 - 200 A and when connecting the welding cable to the left terminal (+) with the full number of turns of the series demagnetizing winding; 160 - 340 A - when connected to the right terminal (+) with part of the turns of the series winding. A rheostat of the RU-3b type with a resistance of 2.98 Ohm for currents of 4.5 - 12 A is included in the circuit of the magnetizing excitation winding, designed to regulate the welding current.
Converter PSG-300-1 is designed to power the post of semi-automatic welding in shielding gas. The converter generator has a rigid external characteristic, which is created by the biasing action of the series field winding. The independent excitation winding is powered by a selenium rectifier connected to the AC mains through a ferroresonant stabilizer. A rheostat is included in the independent excitation winding circuit, which allows you to smoothly adjust the voltage at the generator terminals from 16 to 40 V. The converter is connected to the network by a package switch. Limits of regulation of welding current 75 - 300 A.
Universal welding transducers PSU-300, PSU-500 have both falling and rigid external characteristics. Converters of this type consist of a single-station DC welding generator and a drive three-phase asynchronous motor with a squirrel-cage rotor, located in one housing.
The welding generator of the GSU type is manufactured with four main and two additional poles (Fig. 122). On the two main poles, the turns of the main magnetizing excitation winding are laid, which is powered from the network through a stabilizing transformer and a selenium rectifier. On the other two main poles, turns of a series excitation winding are laid; the magnetic flux of these poles is directed towards the main magnetizing flux. Windings of additional poles are designed to improve switching.
To obtain steeply falling external characteristics, an independent excitation winding, a series demagnetizing winding and part of the winding turns of additional poles are switched on.
When switching to rigid external characteristics (Fig. 122, b) the series demagnetizing winding is partially turned off, but an increased number of turns of the winding of additional poles is turned on.
Changing the type of characteristic is carried out by switching the package switch installed on the switchgear and connecting the welding wires to the two corresponding clamps on the terminal board.
In many cases, installations are used to perform welding work, the main components of which are a step-down transformer, but there are other types of welding equipment. Mostly only professionals know what a welding converter is, but there are many processes in which their use is the only possible option.
Structural device
Welding converter is electric machine, consisting of a drive motor and a generator, which provides the generation of the current necessary to perform the work. Due to the fact that the device of the welding generator includes rotating parts, its efficiency and reliability are somewhat lower than those of traditional rectifiers and transformers.
But the advantage of the converter is that it generates a welding current that is practically independent of the supply voltage fluctuations. Therefore, its use is expedient for performing welding work, to which high quality requirements are imposed.
All working units of the welding converter, including ballasts, are mounted in one single housing. At the same time, there are mobile welding converters and units, as well as stationary posts. The former are mainly used in the performance of installation and construction work, the latter, in the factory.
Installations of this type can generate significant welding current (up to 500 A or more), but it is worth remembering that operation in modes exceeding the standard for this parameter is not allowed. Operation in critical modes can lead to failure of the installation.
ConverterPSO 500
The principle of operation of the welding converter allows you to generate direct and alternating welding current. Very often in production you can see the PSO 500 converter, which is distinguished by high reliability and performance.
Its features include the following points:
The PSO 500 welding converter is mounted on a wheel base, which provides it with good mobility. Thanks to this, the unit can be operated in the conditions of a construction or installation site.
When operating welding converters, it is necessary to follow the rules for the safe operation of electrical equipment:
- The body of the unit must be grounded without fail, all work on connecting the unit to the mains must be carried out by a qualified electrician.
- Given that the converter must be connected to a 220/380V network, the motor terminal box must be securely insulated and covered.
Despite the fact that the welding converter consumes more energy to perform work (due to the presence of mechanical bonds and low efficiency), it provides a stable welding current, independent of power voltage fluctuations, which improves the quality of the weld.
A specific type of welding machine, used mainly in industry, as well as in some types of construction and installation work, is a welding converter.
It is called so because it converts alternating current from a household or industrial network into direct current, which is optimal for most types of welding.
Despite the essence of the end result - direct current - the converter operates on a completely different principle than a rectifier or inverter.
Its design assumes an extended chain of energy passage. First, the alternating current passes into mechanical energy, and it, in turn, is converted back into electrical, but of a permanent nature.
Structurally, the converter consists of an electric motor, usually asynchronous, and a DC generator, combined in one housing. Since the generator, using the principle of electromagnetic induction, also produces alternating current, there is a collector in the circuit that converts it to direct current.
Equipment example
As an example, we can consider the PSO-500 welding converter, widely known in professional circles.
It consists of a cigar-shaped body, on which a block with control equipment, control elements (a batch switch and a rheostat regulator) and contacts for connecting electrodes is fixed on top, and an asynchronous motor and a generator are mounted inside on one rotating shaft, separated by a cooling fan.
No direct electrical connection between generator and engine. The engine, started from the mains, begins to rotate the shaft with which its rotor is connected at high speed.
The generator armature is also mounted on this shaft. As a result of the rotation of the armature, an alternating current is induced in its windings, which is converted into a direct current by the collector and fed to the welding terminals.
PSO-500 refers to single-station welding transducers of mobile type. It is mounted on a three-wheeled trolley. The value of the welding current issued by the PSO-500 can reach 300 or 500 A - depending on the jumper connecting one of the terminals with the serial winding of the generator.
The output current is adjusted manually using a vernier connected to a rheostat (resistance change device). The current is controlled by the built-in ammeter.
The numerical index in the marking - 350, 500, 800, 1000 - means the maximum direct current that this converter is designed to work with. Some models with the help of a vernier can be configured to produce a welding current greater than the rated current, but working in this mode is fraught with overheating and a quick failure of the machine.
Advantages
Like any other equipment, welding converters (which historically appeared much earlier than inverters) have certain advantages, and at the same time bear a number of certain disadvantages. Their advantages include:
- high welding current - for some models, in particular, PSO-500 and PSG-500, it reaches 500 A, there are also more powerful devices;
- unpretentiousness in work;
- insensitivity to input voltage drops;
- relatively high reliability with qualified service;
- good maintainability, serviceability.
The current that these devices are capable of delivering can weld very thick seams, on the order of 10-30 mm. It's another one important advantage, thanks to which welding converters are used.
Flaws
However, design features also determine the main disadvantages of welding converters, due to which, at least in the domestic sphere (welding work in small businesses, in the country, in the garage), they were replaced by inverters. First of all it is:
- large dimensions and weight (it can reach half a ton or more);
- low efficiency;
- increased electrical hazard;
- noise of work;
- the need for service.
The principle of their operation is the transition electrical energy into mechanical and vice versa - implies high energy costs for the rotation of the shaft. This is due to the very high power consumption, which makes the device unprofitable for "home" use.
In addition, the presence of high-speed rotating parts reduces the degree of reliability of the machine. The bottleneck of the brewing converter, as well as the electric motor itself, are ball bearings on which the shaft is fixed.
They need periodic inspection and oil change 1-2 times a year. It is also necessary to monitor the condition of the collector and current collector brushes.
Under the increased electrical hazard, we mean the fact that before starting welding work, the converter must be grounded, according to the rules, it must be connected to the network only by an electrician.
Classification
Welding converters are classified according to various parameters. Including by quantity (single and multi-station) and by type of drive (from an electric motor or, for example, from an internal combustion engine). By design, they can be stationary and mobile, in a single or double housing.
Converters also differ in the shape of the output characteristic. For many types of work, this classification is crucial. According to the shape of the output characteristic, welding transducers are divided into devices that give out a falling or hard characteristic (the latter are also capable of producing a gently dipping one).
There are also universal converters, depending on the installed switch, capable of operating in both modes.
The fact is that the specificity of welding in shielding gases, automatic or semi-automatic, requires an extremely rigid output characteristic.
Such converters include, for example, the PSG-500 system. Welding converters model range PSO have a falling characteristic, PSU are generalists capable of switching to the desired mode of operation.
PSO and other types of converters with a falling characteristic are used in industry, in automatic and manual welding systems equipped with automatic voltage regulators.
From the point of view of applied physics, converters are also divided depending on the technology implemented in the generator. The generator can be with split poles, with separate magnetizing and demagnetizing windings, with demagnetizing winding and independent excitation. But in practice, a significant difference in significant technical specifications between all these types is not.