Green manufacturing is an important strategy for the sustainable development of the machinery manufacturing industry. Normalizing and annealing in the pre-heat treatment of fasteners are the two most commonly used processes, accounting for about 30% of the heat treatment of fasteners. Although these two process methods have their own different characteristics in process, they can achieve the same (or similar) process goals when dealing with low carbon steel or medium carbon steel materials.
According to GB/T16923-1997 “Normalizing and Annealing of Steel Parts”, it can be seen that the normalizing and annealing process methods are divided into a variety of sub-categories. Among them, normalizing and incomplete annealing, isothermal annealing process processing objects, technical requirements Most identical (or similar).
According to the low-carbon steel or medium-carbon steel used in the fasteners, normalizing is to heat the steel to 30～70℃ above Ac3, and then air-cooled or air-cooled to room temperature; incomplete annealing or isothermal annealing is to heat the steel to Ac1 is above 30～50℃, after holding for a certain period of time, it is cooled to a certain temperature with the furnace and then air cooled to room temperature. They are similar or similar in that they can treat medium carbon steel and low carbon steel, and the metallographic structure obtained by the treatment is ferrite + pearlite, which can be used as a preliminary heat treatment or final heat treatment under certain circumstances. So as to achieve the purpose of refining the structure, improving the mechanical properties and cutting performance, and eliminating internal stress.
2. Comparison of process characteristics
a. The grains after normalizing are smaller than the annealed ones, and more pearlite is obtained, and the mechanical properties are slightly higher than that of annealing;
b. Normalizing parts are often charged into the furnace, the heating temperature is generally slightly higher than that of annealing, and the holding time is shorter than that of annealing;
c. Normalizing can eliminate the network structure of carbides, but annealing cannot;
d. Normalizing treatment of carbon steel with lower carbon content is better than annealing, on the contrary, annealing is better than normalizing.
3. Process cost
The comparison between normalizing and annealing mainly exists in the power consumption of resources, the difference in heat preservation and cooling in time, and the corresponding cost difference.
The energy cost is 0.542Kw.h.Kg for normalizing and 0.580Kw.h.Kg for the domestic heat treatment industry evaluation unit consumption value, and the industrial electricity cost is calculated at 0.70 yuan/Kw.h. The average cost of annealing power is 0.03 yuan. /Kg.
The labor cost is about 1 yuan/Kg according to the lowest price of normalizing processing in various places in China. The labor cost of the normalizing process is about 0.32 yuan/Kg, and the labor cost of the annealing process is about 0.35 yuan/Kg, and the difference is 0.03 yuan. /Kg or so.
The price difference between normalizing and annealing heat treatment in the same area in China is about 0.2 yuan/Kg. In fact, after a specific analysis of the price difference, it can be known that the price difference is still based on the traditional heating method and the larger production batches.
4. Process evaluation
When the requirements of manufacturing fasteners and processing technology are basically the same, the normalizing process is more green than the annealing process, which is mainly reflected in the energy consumption and processing man-hours.
Low temperature annealing (stress relief annealing): heating temperature Ac1 carbon steel 550～650℃, eliminate the internal stress in the upsetting and cutting process, and make it reach a stable state.
Recrystallization annealing: heating temperature TR+150～250℃, holding time 0.5～1h, air cooling, recovering recrystallization process occurs, making deformed grains into small equiaxed grains, eliminating cold work hardening effect and internal stress. The annealing temperature for recrystallization of low carbon steel is 600～650℃ and the hardness is in the range of 75～90HRB.
Incomplete annealing: heating temperature Ac1+30～50℃, carbon steel is generally between 700～750℃, to refine grain, reduce hardness, improve plasticity and remove internal stress.
Spheroidizing annealing: The heating temperature is slightly higher than Ac1, and then slowly cooled to less than 500℃ air cooling after long-term heat preservation to spheroidize the carbides and reduce the hardness to improve the cold upsetting performance.
Normalizing: Low-carbon and medium-carbon steel heating temperature Ac3+ (50～70℃), low-carbon steel improves hardness, which is good for cutting, medium-carbon steel refines grains, and uniformly organizes stress relief.
For example: 20# steel three-point welded gasket, using 850℃×0.5h normalizing and 720℃×3h incomplete annealing, the hardness is basically similar to 125～165HBW, normalizing saves production cycle and high efficiency than incomplete annealing.
SWRM15 steel single-sided rivets are normalized at 840℃×0.5h and recrystallized and annealed at 600℃×4h. The hardness of the former is greater than 95HRB, and the hardness of the latter is 78～88HRB which meets the requirements of riveting performance.
For 45 steel large washers, when the hardness requirement is greater than 200HV, normalizing at 870℃×1h is better than incomplete annealing at 760℃×2.5h. The hardness can reach more than 200HV, and the productivity is significantly higher than annealing.
According to statistics, the cost difference between normalizing and annealing processes is 0.2 yuan/Kg～0.45 yuan/Kg. If the normalizing process is properly selected to replace the annealing process, it can bring considerable benefits to fastener companies while achieving green manufacturing. Economic benefits.
In short, with the development of the fastener industry and the promotion of advanced technology, energy saving and consumption reduction will be in the first place, and the green characteristics of the normalizing process will be more obvious and prominent than the annealing process.