Stainless steel or aluminum? Aluminum or stainless steel, which is better Fireproof properties of stainless steel and aluminum
When choosing metal products - heated towel rails and railings, dishes and fences, grates or handrails - we choose, first of all, the material. Traditionally, stainless steel, aluminum and regular black steel (carbon) are considered to compete. Although they have a number of similar characteristics, they nevertheless differ significantly from each other. It makes sense to compare them and figure out which is better: aluminum or stainless steel(black steel, due to its low corrosion resistance, will not be considered).
Aluminum: characteristics, advantages, disadvantages
One of the lightest metals that are generally used in industry. Conducts heat very well and is not subject to oxygen corrosion. Aluminum is produced in several dozen types: each with its own additives that increase strength, oxidation resistance, and malleability. However, with the exception of very expensive aircraft aluminum, they all have one drawback: excessive softness. Parts made of this metal are easily deformed. That is why it is impossible to use aluminum where, during operation, the product is exposed to high pressure (water hammer in water supply systems, for example).
Corrosion resistance of aluminum somewhat overpriced. Yes, metal does not “rot”. But only due to the protective layer of oxide, which forms on the product in air in a matter of hours.
Stainless steel
The alloy has practically no disadvantages - except for the high price. It is not afraid of corrosion, not theoretically, like aluminum, but practically: no oxide film appears on it, which means that over time, “ stainless steel"does not fade.
Slightly heavier than aluminum, stainless steel handles impacts well, high pressure and abrasion (especially brands that contain manganese). Its heat transfer is worse than that of aluminum: but thanks to this, the metal does not “sweat” and there is less condensation on it.
Based on the results of the comparison, it becomes clear that to perform tasks that require low metal weight, strength and reliability, stainless steel is better than aluminum.
Today, aluminum is used in almost all industries, from the production of food utensils to the creation of spacecraft fuselages. For one or another production processes Only certain grades of aluminum are suitable, which have certain physical and chemical properties.
The main properties of the metal are high thermal conductivity, malleability and ductility, resistance to corrosion, low weight and low ohmic resistance. They are directly dependent on the percentage of impurities included in its composition, as well as on the production or enrichment technology. In accordance with this, the main grades of aluminum are distinguished.
Types of aluminum
All metal grades are described and included in unified system recognized national and international standards: European EN, American ASTM and international ISO. In our country, aluminum grades are defined by GOST 11069 and 4784. All documents are considered separately. At the same time, the metal itself is divided into grades, and alloys do not have specifically defined signs.
In accordance with national and international standards, two types of microstructure of unalloyed aluminum should be distinguished:
- high purity with a percentage of more than 99.95%;
- technical purity, containing about 1% impurities and additives.
Compounds of iron and silicon are most often considered as impurities. IN international standard ISO has a separate series for aluminum and its alloys.
Aluminum grades
The technical type of material is divided into certain grades, which are assigned to the relevant standards, for example AD0 according to GOST 4784-97. At the same time, the classification also includes high-frequency metal, so as not to create confusion. This specification contains the following grades:
- Primary (A5, A95, A7E).
- Technical (AD1, AD000, ADS).
- Deformable (AMg2, D1).
- Foundry (VAL10M, AK12pch).
- For deoxidation of steel (AV86, AV97F).
In addition, there are also categories of alloys - aluminum compounds that are used to create alloys from gold, silver, platinum and other precious metals.
Primary aluminum
Primary aluminum (grade A5) - typical example this group. It is obtained by enriching alumina. The metal is not found in nature in its pure form due to its high chemical activity. Combining with other elements, it forms bauxite, nepheline and alunite. Subsequently, alumina is obtained from these ores, and from it, using complex chemical and physical processes, pure aluminum is obtained.
GOST 11069 establishes requirements for grades of primary aluminum, which should be noted by applying vertical and horizontal stripes indelible paint various colors. Found this material wide application in advanced industries, mainly where high technical characteristics are required from raw materials.
Technical aluminum
Technical aluminum is a material with a percentage of foreign impurities of less than 1%. Very often it is also called undoped. Technical brands aluminum according to GOST 4784-97 are characterized by very low strength, but high corrosion resistance. Due to the absence of alloying particles in the composition, a protective oxide film quickly forms on the metal surface, which is stable.
Grades of technical aluminum are distinguished by good thermal and electrical conductivity. Their molecular lattice contains virtually no impurities that scatter the flow of electrons. Thanks to these properties, the material is actively used in instrument making, in the production of heating and heat exchange equipment, and lighting items.
Wrought aluminum
Deformable aluminum includes a material that is subjected to hot and cold pressure treatment: rolling, pressing, drawing and other types. As a result of plastic deformations, semi-finished products of various longitudinal sections are obtained from it: aluminum rod, sheet, strip, plate, profiles and others.
The main brands of deformable material used in domestic production are given in regulatory documents: GOST 4784, OCT1 92014-90, OCT1 90048 and OCT1 90026. Characteristic feature The deformable raw material is a solid solution structure with a high content of eutectic - a liquid phase that is in equilibrium with two or more solid states of matter.
The scope of application of deformable aluminum, like that where aluminum rod is used, is quite extensive. It is used both in areas requiring high technical characteristics from materials - in ship and aircraft construction, and in construction sites as an alloy for welding.
Cast aluminum
Aluminum foundry grades are used for the production of shaped products. Their main feature is a combination of high specific strength and low density, which allows casting of products complex shapes without the formation of cracks.
According to their purpose, foundry grades are conventionally divided into groups:
- Highly hermetic materials (AL2, AL9, AL4M).
- Materials with high strength and heat resistance (AL 19, AL5, AL33).
- Substances with high anti-corrosion resistance.
Very often the performance characteristics of cast aluminum products increase various types heat treatment.
Aluminum for deoxidation
The quality of manufactured products is also influenced by what aluminum has physical properties. And the use of low-grade materials is not limited to the creation of semi-finished products. Very often it is used to deoxidize steel - remove oxygen from molten iron, which is dissolved in it and thereby increases mechanical properties metal For this process the most commonly used brands are AB86 and AB97F.
Currently, the most common NVF systems on the Russian market can be divided into three large groups:
- systems with sub-cladding structures made of aluminum alloys;
- systems with a sub-cladding structure made of galvanized steel with polymer coating;
- systems with sub-cladding structure made of stainless steel.
Undoubtedly, sub-cladding structures made of stainless steel have the best strength and thermal properties.
Comparative analysis of physical and mechanical properties of materials
*The properties of stainless steel and galvanized steel differ slightly.
Thermal and strength characteristics of stainless steel and aluminum
1. Considering the 3 times lower load-bearing capacity and 5.5 times the thermal conductivity of aluminum, the aluminum alloy bracket is a stronger “cold bridge” than the stainless steel bracket. An indicator of this is the coefficient of thermal uniformity of the enclosing structure. According to research data, the coefficient of thermal uniformity of the enclosing structure when using a stainless steel system was 0.86-0.92, and for aluminum systems it is 0.6-0.7, which makes it necessary to lay a greater thickness of insulation and, accordingly, increase the cost of the facade .
For Moscow, the required heat transfer resistance of walls, taking into account the coefficient of thermal uniformity, is for a stainless bracket - 3.13/0.92=3.4 (m2.°C)/W, for an aluminum bracket - 3.13/0.7= 4.47 (m 2 .°C)/W, i.e. 1.07 (m 2 .°C)/W higher. Hence, when using aluminum brackets, the thickness of the insulation (with a thermal conductivity coefficient of 0.045 W/(m°C) should be taken almost 5 cm more (1.07 * 0.045 = 0.048 m).
2. Due to the greater thickness and thermal conductivity of aluminum brackets, according to calculations carried out at the Research Institute of Building Physics, at an outside air temperature of -27 °C, the temperature on the anchor can drop to -3.5 °C and even lower, because in calculations area cross section aluminum bracket was assumed to be 1.8 cm 2, whereas in reality it is 4-7 cm 2. When using a stainless steel bracket, the temperature on the anchor was +8 °C. That is, when using aluminum brackets, the anchor operates in a zone of alternating temperatures, where moisture condensation on the anchor with subsequent freezing is possible. This will gradually destroy the material of the structural layer of the wall around the anchor and accordingly reduce its load-bearing capacity, which is especially important for walls made of material with low bearing capacity(foam concrete, hollow brick, etc.). At the same time, thermal insulation pads under the bracket, due to their small thickness (3-8 mm) and high (relative to insulation) thermal conductivity, reduce heat loss by only 1-2%, i.e. practically do not break the “cold bridge” and have little effect on the temperature of the anchor.
3. Low thermal expansion of guides. The temperature deformation of aluminum alloy is 2.5 times greater than that of stainless steel. Stainless steel has more low coefficient thermal expansion (10 10 -6 °C -1), compared to aluminum (25 10 -6 °C -1). Accordingly, the elongation of 3-meter guides with a temperature difference from -15 °C to +50 °C will be 2 mm for steel and 5 mm for aluminum. Therefore, to compensate for the thermal expansion of the aluminum guide, it is necessary whole line events:
namely, an introduction to the subsystem additional elements- movable slides (for U-shaped brackets) or oval holes with sleeves for rivets - not rigid fixation (for L-shaped brackets).
This inevitably leads to a more complex and expensive subsystem or incorrect installation (as it often happens that installers do not use bushings or incorrectly fix the assembly with additional elements).
As a result of these measures, the weight load falls only on the load-bearing brackets (upper and lower) and the others serve only as support, which means that the anchors are not loaded evenly and this must be taken into account when designing project documentation, which is often simply not done. In steel systems, the entire load is distributed evenly - all nodes are rigidly fixed - minor thermal expansions are compensated by the operation of all elements in the stage of elastic deformation.
The design of the clamp allows the gap between the plates in stainless steel systems to be from 4 mm, while in aluminum systems - at least 7 mm, which also does not suit many customers and spoils appearance building. In addition, the clamp must ensure free movement of the cladding slabs by the amount of extension of the guides, otherwise the slabs will be destroyed (especially at the junction of the guides) or the clamp will unbend (both of which can lead to the cladding slabs falling out). In a steel system there is no danger of the clamp legs unbending, which can happen over time in aluminum systems due to large temperature deformations.
Fire properties of stainless steel and aluminum
The melting point of stainless steel is 1800 °C, and aluminum is 630/670 °C (depending on the alloy). Fire temperature at inner surface tiles (according to test results of the Regional Certification Center “OPYTNOE”) reaches 750 °C. Thus, when using aluminum structures, melting of the substructure and collapse of part of the facade (in the area of the window opening) may occur, and at a temperature of 800-900°C, aluminum itself supports combustion. Stainless steel does not melt in a fire, therefore it is most preferable according to the requirements fire safety. For example, in Moscow during construction high-rise buildings Aluminum substructures are not allowed to be used at all.
Corrosive properties
Today, the only reliable source on the corrosion resistance of a particular sub-cladding structure, and, accordingly, durability, is the expert opinion of ExpertKorr-MISiS.
The most durable structures are made of stainless steel. The service life of such systems is at least 40 years in an urban industrial atmosphere of medium aggressiveness, and at least 50 years in a conditionally clean atmosphere of low aggressiveness.
Aluminum alloys, due to their oxide film, have high corrosion resistance, but in conditions high content In an atmosphere of chlorides and sulfur, rapidly developing intercrystalline corrosion may occur, which leads to a significant decrease in the strength of structural elements and their destruction. Thus, the service life of a structure made of aluminum alloys in an urban industrial atmosphere of average aggressiveness does not exceed 15 years. However, according to the requirements of Rosstroy, in the case of using aluminum alloys for the manufacture of elements of the substructure of an NVF, all elements must necessarily have an anodic coating. The presence of an anodic coating increases the service life of the aluminum alloy substructure. But when installing a substructure, its various elements are connected with rivets, for which holes are drilled, which causes a violation of the anodic coating in the fastening area, i.e., areas without an anodic coating are inevitably created. In addition, the steel core of an aluminum rivet, together with the aluminum medium of the element, forms a galvanic couple, which also leads to the development of active processes of intergranular corrosion in the places where substructure elements are attached. It is worth noting that often the low cost of a particular NVF system with an aluminum alloy substructure is due precisely to the lack of a protective anodic coating on the system elements. Unscrupulous manufacturers of such substructures save on expensive electrochemical anodizing processes for products.
Galvanized steel has insufficient corrosion resistance from the point of view of structural durability. But after applying the polymer coating, the service life of a substructure made of galvanized steel with a polymer coating will be 30 years in an urban industrial atmosphere of medium aggressiveness, and 40 years in a conditionally clean atmosphere of low aggressiveness.
Having compared the above indicators of aluminum and steel substructures, we can conclude that steel substructures are significantly superior to aluminum ones in all respects.
Description of aluminum: Aluminum does not have polymorphic transformations and has a face-centered cube lattice with a period a = 0.4041 nm. Aluminum and its alloys lend themselves well to hot and cold deformation - rolling, forging, pressing, drawing, bending, sheet stamping and other operations.
All aluminum alloys can be joined spot welding, and special alloys can be welded by fusion and other types of welding. Deformable aluminum alloys are divided into hardenable and non-hardenable heat treatment.
All properties of alloys are determined not only by the method of obtaining a semi-finished workpiece and heat treatment, but mainly chemical composition and especially the nature of the phases that strengthen each alloy. The properties of aging aluminum alloys depend on the types of aging: zone, phase or coagulation.
At the stage of coagulation aging (T2 and T3), corrosion resistance increases significantly, and the most optimal combination characteristics of strength, resistance to stress corrosion, exfoliation corrosion, fracture toughness (K 1c) and ductility (especially in the vertical direction).
The condition of semi-finished products, the nature of plating and the direction of cutting samples are indicated as follows - Legend rolled aluminum:
M - Soft, annealed
T - Hardened and naturally aged
T1 - Hardened and artificially aged
T2 - Hardened and artificially aged according to a regime that provides higher values of fracture toughness and better resistance to stress corrosion
TZ - Hardened and artificially aged according to a regime that provides the highest resistance to stress corrosion and fracture toughness
N - cold-worked (colour-working of sheets of alloys such as duralumin approximately 5-7%)
P - Semi-hardened
H1 - Heavily cold-coloured (sheet cold-working approximately 20%)
TPP - Hardened and naturally aged, increased strength
GK - Hot rolled (sheets, slabs)
B - Technological cladding
A - Normal plating
UP - Thickened cladding (8% per side)
D - Longitudinal direction(along the fiber)
P - Transverse direction
B - Altitude direction (thickness)
X - Chord direction
R - Radial direction
PD, DP, VD, VP, ХР, РХ - Direction of sample cutting used to determine fracture toughness and fatigue crack growth rate. The first letter characterizes the direction of the sample axis, the second - the direction of the plane, for example: PV - the sample axis coincides with the width of the semi-finished product, and the crack plane is parallel to the height or thickness.
Analysis and obtaining samples of aluminum: Ores. Currently, aluminum is produced from only one type of ore - bauxite. Commonly used bauxites contain 50-60% A 12 O 3,<30% Fe 2 О 3 , несколько процентов SiО 2 , ТiО 2 , иногда несколько процентов СаО и ряд других окислов.
Samples from bauxite are taken according to general rules, paying special attention to the possibility of moisture absorption by the material, as well as to the different ratios of large and small particles. The weight of the sample depends on the size of the sample being tested: from every 20 tons it is necessary to select at least 5 kg for the total sample.
When sampling bauxite in cone-shaped stacks, small pieces are broken off from all large pieces weighing >2 kg lying in a circle with a radius of 1 m and taken into a shovel. The missing volume is filled small particles material taken from the side surface of the tested cone.
The selected material is collected in tightly closed vessels.
All sample material is crushed in a crusher to particles of 20 mm in size, poured into a cone, reduced and crushed again to particles of size<10 мм. Затем материал еще раз перемешивают и отбирают пробы для определения содержания влаги. Оставшийся материал высушивают, снова сокращают и измельчают до частиц размером < 1 мм. Окончательный материал пробы сокращают до 5 кг и дробят без остатка до частиц мельче 0,25 мм.
Further preparation of the sample for analysis is carried out after drying at 105° C. The particle size of the sample for analysis must be less than 0.09 mm, the amount of material is 50 kg.
Prepared bauxite samples are very prone to stratification. If samples consisting of particles of size<0,25 мм, транспортируют в сосудах, то перед отбором части материала необходимо перемешать весь материал до получения однородного состава. Отбор проб от криолита и фторида алюминия не представляет особых трудностей. Материал, поставляемый в мешках и имеющий однородный состав, опробуют с помощью щупа, причем подпробы отбирают от каждого пятого или десятого мешка. Объединенные подпробы измельчают до тех пор, пока они не будут проходить через сито с размером отверстий 1 мм, и сокращают до массы 1 кг. Этот сокращенный материал пробы измельчают, пока он не будет полностью проходить через сито с размером отверстий 0,25 мм. Затем отбирают пробу для анализа и дробят до получения частиц размером 0,09 мм.
Samples from liquid fluoride melts used in the electrolysis of molten aluminum as electrolytes are taken with a steel scoop from the liquid melt after removing solid deposits from the surface of the bath. A liquid sample of the melt is poured into a mold and a small ingot measuring 150x25x25 mm is obtained; then the entire sample is crushed to a laboratory sample particle size of less than 0.09 mm...
Aluminum smelting: Depending on the scale of production, the nature of casting and energy capabilities, smelting of aluminum alloys can be carried out in crucible furnaces, in resistance electric furnaces and in induction electric furnaces.
Melting aluminum alloys should ensure not only high quality of the finished alloy, but also high productivity of units and, in addition, minimal casting costs.
The most progressive method of melting aluminum alloys is the method of induction heating with industrial frequency currents.
The technology for preparing aluminum alloys consists of the same technological steps as the technology for preparing alloys based on any other metals.
1. When carrying out smelting on fresh pig metals and alloys, aluminum is first loaded (in whole or in parts), and then the alloys are dissolved.
2. When carrying out smelting using a preliminary pig alloy or pig silumin in the charge, first of all the pig alloys are loaded and melted, and then the required amount of aluminum and alloys are added.
3. In the case when the charge is composed of waste and pig metals, it is loaded in the following sequence: pig primary aluminum, defective castings (ingots), waste (first grade) and refined remelt and alloys.
Copper can be introduced into the melt not only in the form of an alloy, but also in the form of electrolytic copper or waste (introduction by dissolution).
