Application of Sensors in Monitoring H2 and CO Concentrations during Steel Quenching
Application of Sensors in Monitoring H2 and CO Concentrations during Steel Quenching
Workpiece heating is carried out in a specific medium. Common heating media include air, fuel gas, salt bath and protective atmosphere.
(1) Air
Air serves as the heating medium when heating in ordinary box-type furnaces or pit-type furnaces. The disadvantage of using air as a heating medium is that oxygen, carbon dioxide and water vapor in it can easily cause oxidation and decarburization of workpieces.
Oxidation refers to the chemical reaction between iron atoms or alloy atoms on the steel surface and oxidizing substances in the heating medium to form oxides. The formation of iron oxide (commonly known as scale) will cause burning loss on the workpiece surface during heating, affecting the surface roughness; during quenching, it will affect the uniformity of quenching cooling, resulting in uneven hardness or insufficient hardness.
Decarburization refers to the phenomenon that carbon on the steel workpiece surface reacts with oxygen, carbon dioxide, water vapor and hydrogen in the heating medium, leading to a reduction in the carbon content on the surface. Decarburization lowers the carbon content on the workpiece surface, resulting in insufficient quenching hardness, which not only reduces its wear resistance but also generates tensile stress on the surface, causing a decrease in fatigue strength.
Oxidation and decarburization seriously affect the quality of heat treatment and should be emphasized and prevented during the heating process.
When air is used as the heating medium, the common methods to prevent oxidation and decarburization are as follows:
Sprinkling methodAfter the furnace temperature reaches the working temperature, load the workpiece into the furnace and immediately spray or sprinkle QW-F1 steel heating protectant into the furnace; alternatively, immerse the workpiece in the heating protectant before loading it into the furnace.
Packing methodPut the workpiece into an iron box, fill it with cast iron chips, charcoal or coke, seal it with a cover, and then place it into the furnace for heating.
Dipping methodMix borax with a mass fraction of 6% and alcohol with a mass fraction of 94% to form a borax-alcohol coating. Dipping the workpiece in the coating before loading it into the furnace can prevent oxidation.
Coating methodApply an anti-oxidation coating on the workpiece surface to isolate it from furnace gas, thereby preventing oxidation and decarburization. Common anti-oxidation coatings are mainly composed of silicates (such as potassium silicate) and metal oxides in a certain proportion (such as chromium oxide, aluminum oxide, silicon oxide). After mixing, binders (such as water glass, resin, silica sol) and diluents (such as water, acetone) are added, and the mixture is adjusted into a paste. The paste is adhered to the workpiece surface by spraying, brushing or dipping to isolate the workpiece from oxidizing atmosphere. Finished anti-oxidation coatings can also be purchased, such as medium-temperature anti-oxidation coating, high-temperature anti-oxidation coating, 110 anti-oxidation coating, 202 anti-oxidation coating, etc.
(2) Fuel gasThe gas generated by the combustion of gas, coal or oil in a flame furnace during heating is called fuel gas. Its main components include carbon dioxide, carbon monoxide, hydrogen and other gases, as well as nitrogen and oxygen from the air. Hydrogen in it can cause decarburization of workpieces, while carbon dioxide and oxygen can cause both oxidation and decarburization of workpieces.
The volume fraction of oxygen in furnace gas is generally controlled between 2% and 4%. A simple estimation method is as follows: put a square wooden block with a side length of about 20mm into the furnace, close the furnace door, and observe the combustion of the wooden block through the observation hole on the furnace door. If the wooden block has no open flame and only smolders and carbonizes, the volume fraction of oxygen in the furnace gas is less than 1.5%; if the wooden block smolders with flickering blue flames, the oxygen volume fraction is 2.5% to 4%; if the wooden block burns with a stable yellow flame and the remaining carbon block keeps flickering, the oxygen volume fraction is 5% or slightly higher. When the oxygen content in the furnace gas is high, the air supply volume can be appropriately reduced to lower the oxidizability of the furnace gas and reduce the oxidation and decarburization of workpieces.
(3) Protective atmosphereThe furnace gas that can protect workpieces from oxidation and decarburization during heat treatment heating is called protective atmosphere. Heating workpieces in a protective atmosphere can obtain a bright surface without oxidation and decarburization, greatly improving both the appearance and internal quality of the workpieces. Common protective atmospheres are as follows:
Controlled atmosphere
Controlled atmosphere refers to the furnace gas whose composition can be adjusted. By regulating the proportion of a certain gas in the furnace gas, the furnace gas can realize non-oxidizing heating, and can also be used for carburizing or carbonitriding.
① Exothermic controlled atmosphereIt is obtained by mixing raw gas (mainly liquefied petroleum gas containing propane C3H8 and butane C4H10, natural gas mainly composed of methane CH4, or city gas) with air in a certain proportion and allowing the mixture to burn on its own. It is called exothermic controlled atmosphere because a large amount of heat is released during combustion. Due to the high content of CO2 and H2O in exothermic atmosphere, it cannot prevent decarburization, and is mostly used for non-oxidizing heating of medium and low carbon steel or non-oxidizing heating that allows a small amount of decarburization.
② Endothermic atmosphereIt is prepared by mixing raw gas (liquefied petroleum gas, city gas or natural gas) with air in a certain proportion and heating the mixture from an external heat source. It is called endothermic atmosphere because the reaction process requires absorbing heat. Endothermic atmosphere contains a high content of CO and H2 but a very low content of CO2 and H2O, so it can effectively prevent oxidation and decarburization of workpieces. To ensure that steel neither decarburizes nor carburizes during heating, the furnace gas composition must be accurately controlled to achieve a balance in the carbon atom exchange between the furnace gas and the steel. At this time, the carbon content of the steel is the carbon potential of the furnace gas. Usually, the carbon potential of furnace gas can be controlled by adjusting the volume fraction of CO2 or H2O in the furnace gas, because the volume fractions of H2O and CO2 have a certain equilibrium relationship with those of CO, H2 and other gases. The content of H2O in furnace gas can be detected by measuring the dew point. The dew point is the temperature at which water vapor in the furnace gas begins to condense into water. The higher the content of H2O in the furnace gas, the higher the dew point, and vice versa. The dew point can be measured with a dew point meter. The content of CO2 can be detected with an infrared CO2 analyzer and controlled by electronic devices. In addition, endothermic atmosphere can also be used for carburizing or carbonitriding.
③ Drop-feed controlled atmosphereHydrocarbons such as kerosene, benzene and methanol are directly dripped into the high-temperature furnace chamber to crack at high temperature. The resulting furnace gas contains reducing gases such as CO, H2 and CH4, which can protect workpieces from oxidation and decarburization. A large number of active carbon atoms are generated after cracking at high temperature, which act as a carburizer to adjust the carbon potential of the furnace gas. The dripping amount of diluent and carburizer should be determined according to factors such as workpiece material, different stages of carburizing, furnace chamber size and furnace load.
Ammonia decomposition gas
Ammonia decomposes into nitrogen and hydrogen at high temperature, which can replace expensive hydrogen as a protective atmosphere to prevent oxidation and decarburization of steel during heating. It is mainly used for bright quenching and annealing of alloy steel with high chromium content. However, hydrogen in ammonia decomposition gas is flammable and explosive, and must not be mixed with air during use.
(4) Salt bathA salt bath is formed by melting inorganic salts into a liquid at high temperature for heating workpieces. Heating with a salt bath can effectively prevent oxidation and decarburization of workpieces because the workpiece is isolated from air. In addition, salt bath heating has the characteristics of fast heating speed, uniform heating, small deformation and the ability to realize local heating. Salt bath heating is mainly used for quenching heating or preheating of workpieces.
