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 a 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 the calculations, the cross-sectional area of the 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 load-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 a lower coefficient of 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, a number of measures are necessary:
namely, the introduction of additional elements into the subsystem - 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 a support, which means that the anchors are not loaded evenly and this must be taken into account when developing design 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 it is at least 7 mm, which also does not suit many customers and spoils the appearance of the 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). The temperature during a fire on the inner surface of the tile (according to the 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, so it is most preferable for fire safety requirements. For example, in Moscow, during the construction of 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, thanks to the oxide film, have high corrosion resistance, but under conditions of high levels of chlorides and sulfur in the atmosphere, rapidly developing intergranular corrosion can 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.
The properties and quality of steels are assessed by a number of technical characteristics, the main of which are mechanical properties and chemical composition, regulated by the relevant GOSTs and technical specifications.
The main indicators of mechanical properties include: strength, elasticity and ductility, tendency to brittle fracture.
Strength - resistance to external force influences.
Elasticity is the property of restoring its original state after removing the load.
Plasticity is the property of obtaining residual deformation after removing the load.
Brittleness is the destruction of a material at small deformations within the limits of elastic work.
The strength, elasticity and ductility of steel are determined by tensile testing of special samples. The resulting diagram shows the relationship between stress and strain.
The most important indicators of the mechanical properties of steel are the yield strength - (R y), tensile strength (tensile strength - R u) and relative elongation (ε). The yield strength and temporary resistance characterize the strength of steel, relative elongation characterizes the plastic properties of steel.
Tensile Diagram of Aluminum Alloys and Steel
1 - pure aluminum; 2 - AMgb; 3 - ABT1; 4 - D16T; 5 — steel grade VStZ
Until a standard low-carbon steel sample reaches stresses equal to the yield strength, the material works almost elastically. Then it develops large deformations under constant stress. As a result, a yield plateau is formed (the horizontal section of the diagram in the figure above). When the relative elongation reaches 2.5%, the fluidity of the material stops and it can again resist deformation. This stage of steel operation is called cmadueit self-strengthening, in which the material acts as elastoplastic. For other steels, the transition to the plastic stage occurs gradually (there is no yield plateau). The yield strength for them is considered to be the stress at which the residual deformation reaches 0.2%, i.e. σ y = σ 0.2.
The ultimate resistance of a material, which characterizes its strength, is determined by the highest conditional stress during the destruction process (the ratio of the destructive load to the initial cross-sectional area of the sample). This stress is called temporary resistance (tensile strength).
The highest stress in the material, at which the deviation from the linear relationship between stress and strain begins, is called the limit of proportionality σ et.
The tendency of steel to go into a brittle state and its sensitivity to various damages is determined by impact strength tests.
The mechanical properties of steel depend on the temperature at which they operate. When steel is heated to t = 250 °C, its properties change slightly, but with a further increase in temperature, the steel becomes brittle. Negative temperatures increase the brittleness of steel, which is especially important to consider when building in the Far North. Low-carbon steels become brittle at temperatures below minus 45 °C, low-alloy steels - at temperatures below minus 60 °C.
Chemical composition of steel. This composition is characterized by the percentage content of various additives and impurities in it. Carbon increases the yield strength and strength of steel, but reduces ductility and weldability. In this regard, only low-carbon steels are used in construction. The special introduction of various impurities (alloying additives) into steel improves some of the properties of steel.
Silicon (denoted by the letter C) deoxidizes steel, so its amount increases from boiling to calm steel. It increases the strength of steel, but somewhat impairs weldability, corrosion resistance and significantly reduces impact strength. The harmful effects of silicon are compensated by the increased content of manganese. Manganese (G) - increases the strength of steel, slightly reducing its ductility. Copper (D) - slightly increases the strength of steel and increases its resistance to corrosion, but contributes to the aging of steel. Aluminum (Au) - deoxidizes steel well, neutralizes the harmful effects of phosphorus, and increases impact strength. The introduction of alloying additives such as nickel (N), chromium (X), vanadium (F), tungsten (V), etc. into steel significantly increases the mechanical properties. However, the use of these additives in steels used in engineering structures is limited by their scarcity and high cost.
Some impurities are harmful to steels. Thus, phosphorus sharply reduces the ductility and toughness of steel, making it brittle at low temperatures. Sulfur somewhat reduces the strength of steel and, most importantly, promotes the formation of cracks during welding. Oxygen, hydrogen and nitrogen, entering the molten metal from the air, deteriorate the structure of the steel, increasing its fragility.
Depending on the mechanical properties (σ u, σ у), all steels are conventionally divided into three groups - regular, increased and high strength. For steels of normal strength, low-carbon steels are used, for steels of increased and high strength - low-alloy and medium-alloy steels.
Depending on the requirements for impact testing, low-carbon steel is divided into six categories, for each of which the chemical composition, tensile strength, elongation and cold bend test requirements are standardized.
Low-carbon steels of grade M16S and grade 16D are intended for hydraulic structures, bridges and other particularly critical structures.
High-strength and high-strength steels (low-alloy and medium-alloy) are supplied in accordance with GOST standards and special technical conditions. The names of alloy steel grades to a certain extent reflect their chemical composition. The first two digits show the average carbon content in hundredths of a percent, the following letters of the Russian alphabet indicate alloying additives. The number after the letter shows the content of the additive as a percentage, rounded to whole numbers. If the amount of alloying additives is 0.3-1%, then the figure is not given. The additive content is not less than 0.3%. All high strength and high strength steels are supplied with a guarantee of mechanical properties and chemical composition. Depending on the standardized properties, according to GOST, steels are divided into 15 categories.
Examples of designation: steel 14G2 has an average carbon content of 0.14%, manganese (G) up to 2%; steel 15ХСНД - carbon 0.15%, chromium (X), silicon (C), nickel (N) and copper (D) 0.3-1% each.
In order to save metal, rolled carbon steel grades StZ, StZGSsps and low-alloy steel grades 09G2, 09G2S and 14G2 are supplied in 2 strength groups (for example, VStZsp5-1 and VStZsp5-2). Such steels differ in different rejection levels of yield strength and tensile strength, and in connection with this, calculated resistances. Steels classified in the second strength group have higher design characteristics.
The choice of steel grade determines the reliability and cost of the structure, ease of manufacture, the duration of its normal operation, the quantity, volume and cost of work on the maintenance of the structure, including corrosion protection.
The steel grade, if the operating conditions of the structures do not impose special requirements, is selected on the basis of variant design and technical and economic analysis.
The strength of the material is characterized by a low stress, upon reaching which the process of destruction of the sample begins. This stress is called tensile strength or tensile strength.
As the strength of steel increases, the yield area noticeably decreases, and some steels are characterized by its complete absence. This property reduces the reliability of steel, increasing its susceptibility to brittle fracture.
For tension, compression and bending when working in the elastic stage, the calculated resistance R y is determined by the standard value using the formula:
where R yn is the standard value, MPa; γ m - reliability coefficient for the material (1.025-1.15).
In the section on the question What is stronger: Steel or Aluminum? given by the author Say a word the best answer is There is no such concept in physics.
Answer from Maksim[guru]
Definitely an aluminum alloy, in general the strength is almost the same, but the weight of the aluminum alloy is, roughly speaking, like feathers, I have a bike like this, I fly on all the curbs. .
In general, aluminum is 100% sure..
Answer from Curdled milk[master]
Steel
Answer from Karagach bala[guru]
Ass. You can hit it with both steel and aluminum and it won’t break.
Answer from European[guru]
foam rubber
Answer from A. Yu.[guru]
you know my friend I'm 64
much stronger
Answer from Yergey Potasov[guru]
It depends on how to compare, under what conditions and by what criteria.
Curiosity's wheels, despite everything, were made of aluminum alloy.
Answer from QWERTY[guru]
Hardness: of course 300% steel
Answer from Oriy Ivanov[guru]
Steel. It has higher hardness and strength.
Answer from Tester[guru]
Somehow I heard the expression on the box
- armor based on aluminum alloy - according to Zvezda about some kind of infantry fighting vehicle
Answer from Engineer[guru]
In terms of absolute strength, nothing stronger than steel has been invented on a macro scale.
In terms of specific strength (tensile strength/specific gravity), aluminum alloys are superior to steels.
Specific strength is needed for flying, sports and special applications.
Both titanium alloys and composites are far ahead of aluminum alloys in terms of specific strength.
P.S. For reference. The most durable aluminum alloy V96Ts-1 has a strength limit of 730 MPa.
Even in ordinary structural steels, the tensile strength is about 1100-1200 MPa, and high-strength steels are far beyond 1500 MPa.
With the growing popularity of suspended ventilated facades, intense competition has arisen between steel and aluminum subsystems. Buyers of illegal armed groups are primarily concerned with such parameters as reliability, durability, and value for money.
Manufacturers of both steel and aluminum subsystems claim that their products have no equal in these indicators. Who to believe? Which material occupies a more advantageous position in NVF systems – steel or aluminum alloy?
Of course, it is wrong to say that steel or aluminum is better, regardless of the purpose of their use. The advantages of aluminum alloys in the production of, for example, heating batteries are indisputable: in terms of thermal conductivity, this material is among the leaders. But it is better to refuse aluminum cookware. It has a short service life due to rapid deformation, but the main thing is that cooking and storing food in such dishes is harmful to health. Aluminum is very delicate and easily detaches from the walls of the cookware, getting into the food. But stainless steel cookware lasts a long time, is absolutely safe and is an attribute of a healthy diet.
But if everything is clear with the same dishes, then the dispute for leadership in the systems of illegal armed groups continues and even intensifies. For those who doubt the choice of substructure based on the type of material, let us compare their main characteristics.
The purpose of any facade is to make the building aesthetically attractive and protect it from cold and wind, rain and snow. Hinged ventilated facades solve these problems and, moreover, are famous for their durability and efficiency.
So, task number one is to insulate the building. Both steel and aluminum subsystems cope with it perfectly, with only one “but”. Aluminum alloy has a landslide victory over steel in terms of thermal conductivity. Unfortunately, this advantage does NVF systems a disservice: aluminum brackets remove three times more heat from the building than their steel “colleagues.” Therefore, when using aluminum subsystems, the insulation should be approximately 5 cm thicker than when using steel subsystems. Accordingly, insulation in this case will cost more.
The second important issue that interests buyers of ventilation facades is the reliability of the system. This indicator includes parameters such as strength, corrosion resistance, temperature deformation, and fire resistance.
Stainless steel is three times stronger than aluminum alloy, so the load-bearing capacity of steel subsystems is much higher. True, it is possible to equalize the strength characteristics by increasing the thickness of the aluminum elements three times, but in this case the aluminum subsystem is equal in price to the steel one. Moreover, in the thickened version it is bulky and heavy, which narrows its scope of application.
By the way, any aluminum subsystem still partially consists of steel elements. Fire shut-offs must be made of stainless steel due to fire safety requirements. The fact is that the melting point of aluminum is 640°C versus 1800°C for stainless steel. The fire temperature inside residential and public buildings reaches 800-900°C. That is why window frames are made of steel: through them the fire comes out.
However, steel fire shutoffs do little to save aluminum subsystems. The susceptibility of the aluminum alloy to temperature deformation leads to the fact that during a fire the cutoffs jump off. As a result, the fire covers the entire subsystem.
Although, according to the expert opinion, both steel and aluminum subsystems have a fire hazard class of K0, that is, they are not fire hazardous, in practice, structures made of aluminum alloys contribute to the spread of fire. Steel structures, on the contrary, are highly fire-resistant. During a fire, they do not burn or melt and thus stop the fire.
In defense of aluminum subsystems, let’s say that their manufacturers are trying to combat this flaw: they introduce additional elements into the design
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 excels at handling impact, high pressure, and abrasion (especially grades 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.
