Steel file is a kind of hand tool widely used in various industries, and is made of carbon tool steel for a long time. This seriously affects the possibility of file manufacturing industry to adopt a variety of forming processing methods. From the actual use of the file, the file is a kind of hand tool without re-grinding, it requires good cutting performance, chip removal performance, and higher service life. For this reason, the tooth peak of the file should have enough hardness, strength, fatigue resistance, as well as high wear resistance and corrosion resistance, and for its center, it requires better toughness and impact resistance. However, the carbon tool steels used nowadays do not have such characteristics as external strength and internal toughness.
Therefore, for a long time, the vast number of engineers and technicians in the steel filing industry at home and abroad have been exploring new materials, new technology and new technology. As early as 1972, Romania developed a low carbon steel surface liquid carbonitriding method to manufacture files. China also conducted tests on making carbon files from low carbon steel between 1960s and 80s. These methods have not been widely used either because of serious pollution, or because of poor efficiency and low performance. So far, the filing industry at home and abroad still preferred to use T12 or T13. A new process for carbonitriding of low carbon steel files made by a factory in Beijing is worth learning from.
1) salt bath carbonitriding formula and heat treatment process. Medium temperature, high efficiency, non-toxic carbonitriding agent formulation (mass fraction) is:
75%BaCl2+20% NaCl+3.5% Na2CO3+1.5% NaCNO（氰酸钠）
75%BaCl2+20% NaCl+3.5% Na2CO3+1.5% NaCNO (sodium cyanate)
The main chemical reactions in the salt bath are in the working state.
2NaCNO+O2→Na2CO3 +2[N] +CO
2NaCNO+O2 to Na2CO3 +2[N] +CO
4NaCNO to 2NaCN+Na2CO3 +CO+2[N]
2NaCN+Na2CO3 to 2Na2O+2[N]+2[C]+CO
2NaCN+3Na2CO3 to 4Na2O+2[N]+5CO
2CO to CO2+[C]
BaCl2 in salt bath can promote carburization and inhibit the production of NaCN:
2NaCN+BaCl2 to 2NaCl+Ba (CN) 2
Ba (CN) to BaCN2 (cyanamide barium) +[C]
BaCl2 in salt bath can also react directly with Na2CO3 in salt bath to form stable BaCO3 and BaO, and some NaCNO:
BaCl2+Na2CO3 to BaCO3 +2NaCl
BaCN2+Na2CO3 to BaO+2NaCNO
The generated BaCO3 and BaO can be removed by slag removal. The presence of NaCl in the composition is mainly to regulate the melting point of the salt bath, increase the fluidity of the salt bath, and thus enhance the heat transfer process.
共渗温度820~840℃，共渗时间视锉刀规格大小及具体要求而定，长250mm的锉刀加热20~ 30min可得到0.4~0.5mm厚的渗层；渗后直接淬水，低温回火。按上述热处理工艺制造的锉刀的切削性能和力学性能超过T12A钢制造的同规格的锉刀。从金相分析可见，渗层最高碳质量分数达0. 8%~0.9%，大量的极细颗粒的碳氮化合物弥散分布在细针马氏体及残留奥氏体之间，形成了高硬度和高疲劳强度的表面硬化层。
The infiltration temperature is 820 ~ 840 C, the infiltration time depends on the size of the file and the specific requirements. The infiltration layer of 0.4 ~ 0.5mm thickness can be obtained by heating the 250mm file for 20 ~ 30 minutes, and quenching water directly after infiltration and tempering at low temperature. The cutting performance and mechanical properties of the files manufactured according to the above heat treatment process exceed those of the same specification manufactured by T12A steel. The metallographic analysis shows that the highest carbon content of the carburized layer is 0.8%~0.9%. A large number of ultra-fine particles of carbonitrides are dispersed between fine-needle martensite and retained austenite, forming a surface hardening layer with high hardness and high fatigue strength.
2) the performance of low carbon steel files. A file is a non regrinding tool. So in terms of regrindability, it is a disposable tool, which requires it to have sufficient cutting performance, chip removal performance, wear resistance, and fatigue resistance.
显然，切削性能、耐磨性能与它的齿面硬度关系极大。图3-3给出了20钢锉刀经上述工艺碳氮共渗后，预冷一直接淬火后得到的齿面硬度（用超声波硬度计检测）与处理时间关系曲线。从图可知，仅浸液15~ 30min，齿面硬度即可达到钢锉所需求的最佳硬度范围63~ 65HRC。
Obviously, cutting performance and wear resistance have great relationship with its hardness. Fig. 3-3 shows the relationship between tooth surface hardness (measured by ultrasonic hardness meter) and treatment time of 20 steel file after carbonitriding and quenching. It can be seen from the diagram that the hardness of tooth surface can reach the optimum range of 63 ~ 65HRC required by steel file only after 15 ~ 30min of immersion.
The cutting performance of the file is closely related to the tooth profile of the cutting teeth. Because of the high original hardness of T12A steel, it is difficult to cut the teeth very deep with a razor blade. The cutting performance can be improved by increasing the tooth depth by 13.3% with low carbon steel.
Relationship between CO diffusion time and hardness
Fig. 3-3 relationship between infiltration time and hardness
表3-3给出了20钢经该工艺碳氮共渗30min所制造出来的锉刀部分性能和目前用的T12A钢锉刀的部分性能对比情况，并列出了世界名牌锉刀DOUBLE FILES和前苏联标准( FOCT)的主要性能。从这些对比数据可以发现，用低碳钢制的锉刀，质量很高，以致超过了世界名牌产品。人们从实践中认识到，锉刀的众多质量指标，综合反映在两点：一是万次锉削试验中的锉屑克数，二是客户的信息反馈，用户满意就是高质量的体现。
Table 3-3 gives a comparison of the partial properties of the file made of 20 steel by carbonitriding for 30 minutes with those of T12A steel, and lists the main properties of the world famous file DOUBLE FILES and Former Soviet Union Standard (FOCT). From these comparative data, it can be found that the quality of the file made of low carbon steel is so high that it exceeds the world famous brand products. It is realized from practice that many quality indexes of file are reflected in two aspects: the number of filing grains in ten thousand filing experiments, and the feedback of customers'information. Customer satisfaction is the embodiment of high quality.
Table 3-3 comparison of file performance of 250mm rough wrench file
Comparison of file performance of 250mm rough wrench file
采用的试块硬度为170~187HBW，截面尺寸为10mm×25mm，压重为12. 5kg的标准锉削试验规范。从实测数据可见，低碳钢碳氮共渗锉刀，其万次锉削锉屑克数不仅比原来的T12A钢锉刀提高近100%，而且超过了名牌DOUB-LE FILES锉刀28%，也超过了前苏联标准。
The hardness of the specimen is 170-187 HBW, the sectional dimension is 10 mm *25 mm, and the pressure is 12.5 kg. From the measured data, it can be seen that the number of cuttings per 10000 times of low carbon steel carbonitriding file is not only 100% higher than the original T12A steel file, but also 28% higher than the famous DOUB-LE FILES file and the former Soviet Union standard.
(1) Compared with the salt bath quenching of T12A steel, the total cost of the file made by carbonitriding process of low carbon steel can be reduced by about 25% with only one item of material cost. The economic benefit is considerable.
(2) the low carbon steel file greatly simplifies the manufacturing process of the file and reduces the total cost by 15%.
(3) The new process will greatly improve the quality of the products and the yield of finished products, so that the rate of first-grade products will be increased from 70% to 90%, and the deformation cracking will be eliminated fundamentally. Thus the total cost will be reduced by about 15%, thus greatly improving the economic benefits of enterprises.
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