Heat treatment and surface modification of 45# steel

45# steel is one of the most widely used steel, almost all heat treatment workers have dealt with it, most colleagues think that 45# steel is a very difficult “opponent”, water quenching is easy to crack, oil quenching is not hard. The steel is mainly used for manufacturing all kinds of forgings. Heavy forgings can be used under normalizing condition, and can also be quenched and tempered; Small forgings are basically quenched and tempered. According to the need, it can be tempered at low or medium temperature after quenching.

With the development of national economy and the demand of export-oriented economy, 45# steel is not only used to manufacture various components in engineering, but also can be used to manufacture low-grade molds, slow cutting taps, dies and other tools. It can also be used to manufacture steel wire pliers, spanners, screwdrivers, hammers and other five metal tools, as well as various garden tools and agricultural machinery tools. The following is a brief introduction of the heat treatment process and surface modification process of 45# steel for reference.

Chemical composition, heat treatment critical point and general process of 45# steel

The chemical composition of 45# steel is shown in Table 1, and the critical point of heat treatment is shown in Table 2.

Table 1 chemical composition of 45# steel (mass fraction) (%)

С

Si

 Mn

P

Ni

Cr

S

Cu

As

Fe

0.42-0.5

0.17-0.37
0.5-0.8
<0.035
<0.25 <0.25
<0.04
<0.25 <0.08
~97

Table 2 heat treatment critical point of 45# steel (℃)

Ac1 Ac2 Ac3 Ac4
724 780 751 300

The general heat treatment process of 45# steel is as follows:

  • 1) Softening treatment: 740 ~ 760 ℃ × 4 ~ 6h, at the cooling rate of 50 ~ 100 ℃ / h, the furnace is cooled to 600 ℃ and discharged for air cooling.
  • 2) Normalizing: heating at 850 ~ 870 ℃, air cooling after discharging.
  • 3) Quenching and tempering: 840 ~ 860 ℃, 450 ~ 550 ℃.
  • 4) Quenching + tempering: 820 ~ 840 ℃ heating quenching, according to the actual situation, select the appropriate coolant; The tempering process depends on the required mechanical properties of the workpiece.

Boiling water quenching of 45# steel

The critical quenching diameter of 45# steel is 10 ~ 15mm when it is cooled by tap water and less than 25mm when it is cooled by salt water. If the workpiece size exceeds the critical quenching diameter, the quenching and tempering effect is poor, and the larger the size is, the worse the effect is. Therefore, the quenching and tempering treatment of large cross-section workpiece of 45# steel will not get uniform structure along the cross-section, which can not achieve the effect of quenching and tempering. In order to improve this situation, it is generally used to increase the heating temperature or select severe cooling medium. The former will cause oxidative decarburization and high energy consumption, while the latter is easy to cause deformation and cracking. Therefore, normalizing is often used as the final treatment for the workpiece quenched beyond the critical diameter, but in addition to the slow cooling speed, the cooling speed is also affected by the environment, air temperature and temperature, so that the hardness can not be controlled, and the quality of normalizing is difficult to guarantee.
The heat treatment process of boiling water quenching instead of quenching and tempering has been successfully studied in China, and the brief introduction is as follows: the WC of 45# steel used in the test is 0.46%, the box furnace is used for heating, and the sample size is 0 φ 50mm × 150mm、 φ 70mm × 210mm、 φ 80mm × 210 mm, heating temperature 840 ℃, using 100 ℃ boiling water cooling, on its 1 / 2 length cross section, along three straight lines passing through the center and evenly distributed.

  • 1) The hardness and microstructure of 45# steel with large cross section quenched in boiling water are relatively uniform along the whole cross section. Because the boiling water is always kept at 100 ℃, the mechanical properties are not affected by the environment and season.
  • 2) As the final heat treatment of the workpiece, the normalizing temperature can be reduced by 15 ~ 20 ℃, and the comprehensive mechanical properties are good.
  • 3) If the cooling water temperature is properly selected, boiling water quenching of large cross-section workpiece of 45# steel can completely replace quenching and tempering treatment, and can shorten the production cycle, save energy and increase efficiency.

Quenching and cooling of 45# steel

Now φ 12.5mm × For example, the WC is 0.46%, and the heating temperature is 840 ℃. The quenching results in different media are discussed.
(1) Water quenching
At the end of quenching at 840 ℃, the transformation of pearlite and ferrite is inhibited due to the rapid cooling rate (about 540 ℃ / s), and the undercooled austenite transforms into martensite at 300 ℃. The properties of this structure are: RM ≥ 1100mpa, rel ≥ 720mpa, a ≥ 7%, Z ≥ 15%, hardness 52 ~ 60HRC.
(2) Oil quenching
After austenitizing at 840 ℃, it is quenched into N32 oil immediately. Due to the slow cooling rate (about 240 ℃ / s), the undercooled austenite does not transform at the beginning. Until 450 ℃, part of the austenite transforms into bainite. When it is cooled to about 300 ℃, the decomposition of austenite stops. When it is cooled further, the rest of the undercooled austenite transforms into martensite. The microstructure at room temperature is less than 20% bainite + 80% martensite + a small amount of retained austenite.
(3) Air cooling
The microstructure of undercooled austenite is 30% ferrite + 70% pearlite at room temperature, and the hardness is 220 ~ 250hbw.
(4) Other quenchants
In order to fully tap the potential of 45# steel, many new quenchants have been developed and introduced from their own products, such as two nitrate aqueous solution, PAG quenchant, three nitrate quenchant, calcium chloride aqueous solution, sodium carbonate aqueous solution, organic solvent and inorganic water-based quenchants.

Quenching crack of 45# steel should not be ignored

When small steel parts are quenched by conventional process, there are “thin, short, shallow and straight” tile shaped cracks, the depth of which is about 1% ~ 10% of the diameter of the quenched workpiece. If the workpiece surface is rough or has scratches, coarse cracks may also appear. This regular “thin, short, shallow and straight” crack feature is the so-called crack.

For many years, people have not clearly understood the nature of crack, but their research has never stopped. The results show that the crack is not directly related to the raw materials, but is related to whether the size of the workpiece is in danger of cracking, and the actual carbon content of the steel also has an impact. Domestic colleagues have done a lot of work, published a very insightful practical article, the author will draw it into figure 1.

20210523013251 73440 - Heat treatment and surface modification of 45# steel

Fig.1 relationship between quenching crack rate of 45# steel and section size of workpiece

It can be seen from Figure 1 that the crack begins at 5mm and increases with the increase of the cross-section size of the workpiece. The peak value is 6 ~ 8mm, and the quenching crack rate is almost 100% at 7mm. The crack tends to be flat after 9mm, and no crack is found at 12mm. This conclusion is accepted by the majority of domestic colleagues. Why does this special “dangerous quenching crack size” appear? The same view is that the surface stress of 45# steel workpiece is tensile stress when it is fully hardened. When the tensile stress exceeds the fracture strength of the material, cracks will initiate from the surface, φ 5mm~ φ This is the reason why 8mm workpiece is easy to crack. When the diameter of workpiece is less than or equal to 4mm, because the cross section is small, the temperature difference between the surface and the center is small, the tensile stress of quenching is also small, and the surface tensile stress does not exceed the fracture strength of the material, so it is not easy to produce cracks; When the diameter is larger than 8 mm, although the quenching residual stress increases due to the increase of cross section, the peak value of tensile stress is far away from the surface of the workpiece, and the surface tensile stress still does not exceed the fracture strength of the material. If the size of the workpiece continues to increase and exceeds the critical quenching diameter of the steel, the surface of the workpiece will show compressive stress when it is not hardened, so it is more difficult to crack.
Another important reason for the cracking of 45# steel is the sudden change of critical point. The main basis for formulating heat treatment process is based on its chemical composition and critical point, while the critical point of steel is mainly determined by carbon content. The range of carbon content in national standard 45# steel is too large, the measured critical point is very different from the nominal critical point in the manual, which brings great difficulties to the formulation of heat treatment process.
Figure 2 is a curve drawn according to the actual situation. It can be seen from Fig. 2 that the critical points AC1 and AR1 of steel do not decrease gently with the increase of carbon content of steel, but there is a large fluctuation in a very small range, that is, the mutation of critical point. For example, the AC3 point of 45# steel with WC of 0.45% ~ 0.50% has a very low value, which is about 760 ℃. The quenching process is still formulated according to 780 ℃ + 50 ℃ in the book, which is obviously problematic.

20210523013537 42333 - Heat treatment and surface modification of 45# steel

How to solve the problem of quenching crack of dangerous size? Now there are various measures:

  • 1) Sub temperature quenching. That is to say, the original quenching temperature is reduced from 830 ~ 840 ℃ to 780 ℃.
  • 2) Work on quenching and cooling medium. Many domestic colleagues, according to their own products, make their own CaCl2, Na2CO3, two nitrate, three nitrate aqueous solutions, or buy the products of Nanjing Kerun, Liaoning Hexing and other companies, which not only solve the cracking problem, but also save energy and environmental protection, increase efficiency, and really kill many birds with one stone.

Surface treatment and chemical heat treatment of 45# steel

Blackening treatment of small steel parts:
Formula: NaOH (600 ~ 700g / L) + NaNO2 (150 ~ 250G / L). Working temperature 138 ~ 140 ℃, blackening time 45 ~ 60min.
Blackening quality inspection:

  • 1) Appearance inspection. When the workpiece is placed under the fluorescent lamp, it shall be 300 mm away from the naked eye, and the color shall be uniform black without obvious speckles.
  • 2) Density test. If there is no Cu precipitation after soaking in standard solution CuSO4 · 5H2O for 1 min (room temperature), it is qualified.
  • 3) Anti rust ability inspection. Place the blackened parts in 30 g / L NaCl solution at 20 ℃ for 10 min, and expose them in air above 20 ℃ for 2 h. It is qualified if there is no rust after visual inspection.
  • 4) Corrosion resistance test. Put the blackened parts in the oxalic acid solution of 50 g / L at 20 ℃ for 8 min and take them out. If the surface color is brown, dark gray and dark green, they are qualified.
  • 5) Firmness inspection. If there is no scratch after rubbing 20 times, it is qualified.

Phosphating treatment of 45# steel workpiece

In the 1990s, Zhejiang tool factory and other units produced a large number of 45# steel taps and dies for foreign investors. After quenching and tempering, they need phosphating treatment
30 ~ 35g / L manganese iron dihydrogen phosphate (mariff salt), 80 ~ 100g / L zinc nitrate Zn (NO3) 2 · 6H2O, free acidity 5 ~ 7, total acidity 60 ~ 80, temperature 60 ~ 70 ℃, time 10 ~ 15min.
Inspection of phosphating quality:
1) Appearance inspection. The phosphating film surface should be gray or dark gray, uniform, dense, firm and complete.
2) Corrosion resistance inspection. ① Immersion method. First, immerse the phosphating parts in 3% NaCl aqueous solution and keep it at room temperature for 15min, then take them out and rinse them with tap water, dry them naturally in the air for 30min, or blow dry them quickly with compressed air. If there are no rust spots on the surface, they are qualified. ② Drip method. Absorb a little component solution (71.05g CuSO4 · 5H2O + 132.9g solid NaCl + 0.1N hydrochloric acid 13.2ml + 986ml distilled water) and drop it on the surface of phosphating parts at room temperature without discoloration within 1min.
Since most of the 45# steel workpieces are tempered at a low temperature below 250 ℃ after quenching, the surface treatment temperature should be lower than this temperature. In addition to the above blackening and phosphating, some units also use molybdening treatment and sulfurizing treatment.

Application example of chemical heat treatment for 45# steel workpiece

(1) 45 × steel hot cutting side die
Solid boronizing 910 ~ 930 ℃ × After 4 h solid boronizing, it is cooled to room temperature with furnace; Salt bath quenching, 820 ~ 840 ℃ × 5min, quenched in brine, 210 ~ 230 ℃ × 3 h tempering, surface hardness 1290 ~ 1700hv.
(2) Liquid boronizing of 45# steel mould
Good results can be obtained by liquid boronizing of 45 ﹥ steel seamless steel pipe cold drawing die, silicon rod die, glazed tile template, etc. Common liquid boronizing process: 920 ~ 940 ℃ × 5 ~ 6h, out of furnace quenching, low temperature tempering.
(3) Solid Ni-B co infiltration for 45# steel mould
Ni-B co infiltration is better than single infiltration. The process is electroless Ni plating, forming a compact and smooth coating on the metal surface, and then b-permeating. Due to the mutual diffusion and penetration of Ni, B and Fe, the ideal coating and b-permeating layer can be obtained, which improves the bonding strength, cold and hot fatigue performance, high temperature oxidation resistance and wear resistance, and reduces the brittleness of b-permeating alone.
(4) Electroless deposition of Ni-P alloy
The cold punch of angle steel and steel plate is made of 45# steel, which is quenched at 830 ℃ and tempered at low temperature to obtain hardness of 50 ~ 54hrc. After being processed into finished products, Ni-P alloy is deposited, and the thickness of deposited layer is 25 ~ 30 μ m. The surface hardness can reach 1000hv, the service life of punch can reach more than 6000 pieces, and the average service life can be increased by more than two times.

Concluding remarks

  • 1) Because 45 # steel has the characteristics of critical point mutation, the actual carbon content of 45 # steel must be paid attention to when dealing with small parts.
  • 2) The traditional NaCl solution should not be used for quenching small steel parts. It is suggested to use organic or inorganic solvents which are hard and not easy to crack.
  • 3) Boiling water quenching of 45# steel is a promising technology.
  • 4) Fully tap the potential of 45# steel, 45# steel can manufacture a variety of engineering components and tools.
  • 5) 45# steel parts can be treated with various surface treatments and chemical heat treatments. No matter what kind of treatment is adopted, it is the absolute principle to improve the product quality of the unit.

Author: Zhao Buqing

Source: Steel Parts Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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