Determination of quenching and tempering process p

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Determination of quenching and tempering process parameters of steel

1 preface

quenching is the most effective heat treatment process for strengthening materials, and the selection of process parameters directly affects the properties of materials. This requires the heat treatment workers to constantly innovate, improve the process, give full play to the potential of materials, save energy and reduce production costs. This paper briefly describes the determination and quantitative basis of quenching and tempering process parameters of steel

2 quenching heating temperature

according to the conventional process, the quenching heating temperature of hypoeutectoid steel is AC3 + (30 ~ 50 ℃); For eutectoid and hypereutectoid steels, AC1 + (30 ~ 50 ℃); The quenching heating temperature of alloy steel is often AC1 (or AC3) + (50 ~ 100 ℃); High alloy steels contain a large number of high melting point carbides. To increase the austenitizing degree, the quenching heating temperature is higher, and some have reached the level close to the melting point

in order to achieve the different properties required by steel, the quenching heating temperature is developing to high or low. Sub temperature quenching is to reduce the quenching temperature to 5 ~ 10 ℃ below AC3 α+γ In the two-phase region, a total of 12600 tons of HS code:) 1 are quenched with about 10% ~ 15% undissolved ferrite. While ensuring strength and high hardness, plasticity and toughness are improved, quenching deformation or cracking are significantly reduced, and tempering brittleness is also weakened. As a new mature process, it has been recognized by heat treatment workers at home and abroad

in addition, it has been found that [1] the hypoeutectoid steel represented by 40Cr steel has a hardening peak at point AC3. This temperature quenching can not only obtain the highest hardness, but also have the best mechanical properties. If you master it properly, you can give full play to the potential of the steel

on the contrary, unexpected results can be obtained by increasing the quenching temperature of some steels. For example, the quenching temperature of hot die steel 5CrMnMo and 5CrNiMo steel is increased from the traditional 860 ℃ to 920 ℃ (30 ~ 80 ℃) [2], which accelerates the dissolution of carbides, increases the alloy content in martensite, and has uniform microstructure. A large amount of high dislocation martensite can be obtained, the fracture toughness is greatly improved, the red hardness is more excellent, and its service life is doubled. For another example, when the quenching temperature of H13 steel is increased from 1050 ℃ to 1100 ℃, the austenite grain does not grow obviously. Due to the accelerated dissolution of carbides, the carbon and alloy elements in austenite increase, resulting in δ b、 δ 0.2 (room temperature and 500 ℃) and the improvement of thermal fatigue properties are conducive to prolonging the service life of H13 steel die [ 3 ]

with the different properties required for hypoeutectoid steel, the choice of quenching temperature has great flexibility. However, whether the temperature is increased or decreased, it is mainly based on the critical point AC3 of steel. Therefore, it is very important to correctly master AC3 point of steel. In recent years, heat treatment workers have developed AC3 point calculation model [4]

in recent years, the imported or newly developed low-alloy wear-resistant steels for bucket teeth of construction machinery, such as zg30crmn2sireb steel, are hypoeutectoid steel [5,6]. In order to give full play to the potential of steel and obtain wear resistance and certain strength and toughness, the quenching temperature adopted is 90 ~ 120 ℃ higher than the traditional temperature. This shows that the quenching temperature of steel is very different for different steel types and required properties, which can not be generalized, and it is necessary to jump out of the traditional constraints

The quenching temperature of

high alloy steel also has a great change, from qualitative to quantitative, so that the selected quenching temperature is more practical. Some people put forward the concept of equilibrium carbon [6], and thus determine the normal quenching temperature

equilibrium carbon CS = 0.033ww + 0.063wmo + 0.06wcr + 0.2wv

the carbon saturation a of the steel is the ratio of the actual carbon content C in the steel to the equilibrium carbon CS, that is, a = C real/cs. The optimum quenching temperature can be determined by different a values, and satisfactory quality requirements can be obtained. Some people also put forward a method to determine the quenching temperature of high-speed steel based on the dissolution temperature of carbides, that is,

ts (° f) = 2310-200wc + 40wv + 8WW + 5wmo ± 12

t quenching (° f) = TS - (35-50)

the high-speed steel used to make molds should not only have certain wear resistance, but also have certain toughness. The selected heating temperature should be lower than the traditional one, Generally, it is determined by the following formula [ 7 ]:

w18cr4v steel t (℃) = 1260 - (64 HRC value) × 10


w6mo5cr4v2 steel t (℃) = 1190 - (64 HRC value) × 10


where HRC is the required hardness value of the mold

3 heating time

in order to reduce production cost and improve production efficiency, shortening heating time is an effective and simple method. Through a large number of tests and comparisons, it is found that there are some problems in the traditional method of determining the heating time. It is proposed that the heating time shown in Table 1 is more suitable for practice, which is significantly less than the traditional heating time. Table 1 press τ= KW recommended W value for calculating holding time

workpiece shape w/CMK/- 1 columnar


tubular (1/6 ~ 1/4) Bayer materials technology (China) Co., Ltd. as the main supporting unit d

(1/6 ~ 1/2) b

(1/4 ~ 1/2) δ 7


10 note: for salt furnace heating. D、B、 δ For the heating time of large cross-section workpieces, some people think that the quenching effect of workpieces with large cross-section is only a certain depth. Complete heat penetration during heating not only prolongs the time and wastes energy, but also relatively increases the heat lost in the cooling process, and its cooling intensity decreases, making the actual quenching effect worse. It is found that the austenite phase transformation generally does not exceed a few minutes, so the heating time shall be subject to ensuring that the temperature inside and outside the workpiece section is consistent. Based on this, some people put forward the new concept of zero insulation, which has been gradually accepted by people

4 cooling

in order to make the quenching cooling of steel more suitable, the selection of medium and cooling intensity should be based on the critical cooling rate of steel. Heat treatment workers have derived different types of calculation formulas or models, The representative formula is as follows: [8]

(1) the critical cooling rate for obtaining martensite

lgv1 = 9.81 - (4.26wc + 1.05wmn + 0.54wni + 0.5wcr + 0.66wmo + 0.00183pa)

(2) the critical cooling rate for obtaining bainite

lgv2 = 10.17 - (3.08wc + 1.07wmn + 0.70wni + 0.57wcr + 1.58wmo)+ 0.0032pa) (℃/h)

where pa - austenitizing parameter

due to the influence of the "quenching quality effect" of the workpiece, the actual cooling rate of the workpiece with different cross sections varies greatly. Therefore, someone proposed the quantitative relationship between the cross section and the cooling intensity during water and oil quenching:

in the formula, H1 and H0 are the cooling intensity under different stirring and static states respectively

mold quenching cooling requires a certain amount of residual heat. Some people have summarized the empirical formula [9, 10] that determines the quenching cooling time:

in the formula, a - the state coefficient of oil

v, F - the volume and surface area of the mold respectively, DM3, dm2

d - the height or thickness of the mold, mm

spray cooling quenching solves the problem of insufficient quenching cooling of large cross-section workpieces by adjusting the spray pressure The cooling intensity is controlled by flow and time to realize computer control and meet the needs of mass quenching [11, 12]. In addition, spray quenching can control the cooling of the workpiece to a certain extent, so that it can retain a certain residual temperature and use the residual heat for self tempering. It is energy-saving, time-saving, efficient and has great development potential

5 evaluation of quenching effect

in the past, the hardenability of steel can only be qualitatively checked from the end quenching chart, which is inconvenient to use. In recent years, evaluating the hardenability of steel has been gradually quantified, that is, calculated by the corresponding formula, which is intuitive, convenient and reliable. The typical application formula is as follows [13]:

+ 16wmn + 35wmo + 5wsi-0.82kastm

in the formula, e - distance to quenching end, mm

kastm - grain size grade

it is not enough to use hardness evaluation only for some steel grades, so it must cooperate with structure observation and performance test. For example, for zg30crmn2sireb steel, the process parameters to achieve the highest quenching hardness are not the best. However, the hardness decreases slightly when the quenching temperature is higher than the highest hardness, but the wear resistance and strength toughness are the best

6 tempering

generally, the tempering process parameters of steel are selected from the relevant manuals according to the required hardness and mechanical properties of steel. It is not only troublesome to use, but also difficult to start with new steel types. In order to solve these problems, heat treatment workers have done a lot of work. Based on tempering dynamics, various types of tempering special formulas [14,15] and general formulas [16,17] have been summarized and derived. These formulas provide conditions for on-site production and computerization of process planning

in order to improve production efficiency, a rapid tempering process was developed. The principle of rapid tempering is based on the constraint relationship between tempering parameter P and steel properties and hardness. That is, when the tempering process parameters are equal, the obtained hardness or mechanical properties are basically the same. Tempering parameter p =( θ+ 273) (WC + LGT) is the temperature θ As a function of time t, to obtain the same tempering effect, different θ And T [18]

in the past, the actual effect of repeated fire on the basis of different fixtures has not attracted people's attention, and there are few studies. Literature [19] summarized and proposed to measure the cumulative effect of multiple tempering. For example, the tempering parameters of steel at various temperatures are P1, P2... Respectively, and the cumulative total tempering parameter P can be expressed as:

p total = LG (10p1 + 10p2...)

the quantitative evaluation of the tempering effect at multiple different temperatures can be said to be the deepening and improvement of the understanding of the tempering process


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