When the steel fasteners are heated in various gaseous media, due to the different effects of various gases on the steel, oxidation, decarburization, and other reactions occur on the surface of the steel, and the chemical composition and quality thereof change.
The commonly used controlled atmosphere contains hydrogen, carbon monoxide, methane, carbon dioxide, nitrogen, moisture, and trace amounts of oxygen. To do this, the fasteners should be heated to protect the atmosphere.
At present, mesh belt furnaces use methanol and propane or toluene drip type cracking as the main protective atmosphere. Commonly used are methanol + propane, methanol + toluene, nitrogen, methanol and other organic liquids to crack controllable gas. The nitrogen controlled gas is used in the furnace where the fastener is heated, because it will not cause decarburization, and it can be heated and protected in low carbon steel and medium carbon steel by quenching.
The Co content of this gas is very low, so the price is much cheaper. Production practice has proved that the use of nitrogen and methanol atmosphere can reduce the cost of about 20% to 30%, and the economic benefits are significant.
Methanol is decomposed at temperatures above 800°C as CH3OH → Co+2 H2. Its composition is 1/3 Co and 2/3 H2 with small amounts of CO2, H2O and CH4. With the different reaction temperatures, there is a tendency to form free carbon, which is a kind of gas with weak carburizing or non-carburizing gas and protective gas.
Nitrogen is an inert gas, non-toxic, non-polluting environment, there is no burning explosion risk, called the green heat treatment atmosphere. The main components of nitrogen-methanol atmosphere are N2, H2, and Co, with trace amounts of H2O, CO2, and CH4. By adjusting the mixing ratio of nitrogen and methanol, Co in the furnace atmosphere can vary from 0 to 33%, while the concentration of H2 in the atmosphere is always twice the Co concentration and the balance is N2.
Since methanol is a weak carburizing agent, when the carbon potential in the furnace reaches a certain level (usually 0.35%C to 0.40%C), it is necessary to add a certain amount of energy to the furnace in order to rapidly increase the carbon potential in the furnace to meet production needs. The amount of rich gas is increased by 2% to 5% of propane gas or methanol infused into the furnace to rapidly increase the carbon potential and reach the process set point. The proportion of gas after decomposition in the furnace is basically 20% CO, 40% H2, and 40% N2. It is worth noting that the N2 flow rate is 10-15L/min.
Propane is an inflammable and explosive material that will explode and burn in the event of a strong collision. Toluene is a strong carburizing organic liquid cracking gas. Toluene is an easy-to-produce reagent. Reducing the amount of propane and toluene can effectively reduce unsafe factors and also reduce the harmful effects of harmful gases on employees.
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