阅读说明：本技术 一种球墨铸铁制件表面显微组织梯度化及提高耐磨性的新型淬火-配分-等温热处理工艺 (Novel quenching-partitioning-isothermal heat treatment process for gradient microstructure on surface of nodular cast iron product and improvement of wear resistance ) 是由 董克文 王璇 周文韬 刘澄 于 2020-07-13 设计创作，主要内容包括：本发明涉及一种球墨铸铁制件表面显微组织梯度化及提高耐磨性的新型淬火-配分-等温热处理工艺,包括以下步骤：步骤1)、奥氏体化控制；奥氏体化过程包括两个部分的控制,即奥氏体化过程的参数控制以及球墨铸铁制件的表层碳粉浓度、厚度控制；步骤2)、设置加热炉温度,在球墨铸铁制件表层包覆碳粉,然后将包裹有碳粉的球墨铸铁制件置于坩埚内,此后将坩埚盖盖上；在炉温达到设定温度后,将坩埚置于炉内；保温时间后迅速将球墨铸铁制件放于60-80℃的水基淬火液中进行快速冷却,冷却2-6秒；步骤3)、将淬火后的球墨铸铁制件迅速取出保温处理；步骤4)、将球墨铸铁制件取出,悬置并空冷至室温。通过本发明,使制件的耐磨性得到提高。(The invention relates to a novel quenching-partitioning-isothermal heat treatment process for gradient microstructure and improvement of wear resistance of a surface of a nodular cast iron workpiece, which comprises the following steps: step 1), austenitizing control; the austenitizing process comprises two parts of control, namely parameter control of the austenitizing process and control of the concentration and thickness of carbon powder on the surface layer of the nodular cast iron product; step 2), setting the temperature of the heating furnace, coating carbon powder on the surface layer of the nodular cast iron part, then placing the nodular cast iron part coated with the carbon powder in a crucible, and then covering the crucible with a cover; after the furnace temperature reaches the set temperature, placing the crucible in the furnace; after the heat preservation time, rapidly placing the nodular cast iron workpiece in water-based quenching liquid at the temperature of 60-80 ℃ for rapid cooling for 2-6 seconds; step 3), rapidly taking out the quenched ductile cast iron workpiece for heat preservation treatment; and 4) taking out the nodular cast iron part, suspending and air-cooling to room temperature. By the invention, the wear resistance of the part is improved.)

1. A novel quenching-distribution-isothermal heat treatment process for gradient microstructure and improvement of wear resistance of a nodular cast iron workpiece surface is characterized by comprising the following steps:

step 1), austenitizing control; the austenitizing process comprises two parts of control, namely parameter control of the austenitizing process and control of the concentration and thickness of carbon powder on the surface layer of the nodular cast iron product;

i, controlling parameters of an austenitizing process as follows:

austenitizing temperature (T)_AAnd deg.C) is formulated according to equation (1):

in the formula: t is_AAustenitizing temperature in degrees celsius; % C is the mass fraction of carbon element; % Ni is the mass fraction of nickel element; % Si is the mass fraction of silicon element; % V is the mass fraction of vanadium; % Mo is the mass fraction of molybdenum element; % W is the mass fraction of tungsten element; % Mn is the mass fraction of manganese element; % Cr is the mass fraction of chromium element; % Cu is the mass fraction of copper element; % P is the mass fraction of phosphorus element; % Al is the mass fraction of aluminum element; % A is the mass fraction of arsenic; % Ti is the mass fraction of titanium element;

austenitizing Heat preservation time (t)_AMinutes) is formulated according to equation (2):

t_A＝3H+18 (2)

in the formula: h is the thickness of the nodular cast iron part, and the unit is millimeter; t is t_AAustenitizing holding time in minutes;

II, controlling parameters of the austenitized carbon powder as follows:

the surface layer of the nodular cast iron product is coated with carbon powder, wherein the concentration of the carbon powder is more than or equal to 98.0 percent, and the particle size of the carbon powder is less than or equal to 1 × 10^-6Millimeter;

coating carbon powder on the surface layer of the nodular cast iron part, and calculating the thickness d of the carbon powder layer according to a formula (3):

wherein d is the thickness of the carbon powder layer and the unit is meter, Q is the diffusion activation energy of carbon atoms in austenite, 1.34 × 10⁵Coke/mole; r is a gas constant of 8.314J/(mol)Er (Chinese character of 'Er')On); t is absolute temperature, and the unit is on; d is a diffusion coefficient; wherein D can be determined by calculation of formula (4);

in the formula: c_s＝0.02；C＝0.85C₀；C₀Is the original mass fraction of carbon element in steel,%; t is austenitizing heat preservation time for minutes;

step 2), setting the temperature of a heating furnace according to the austenitizing temperature calculated in the step 1), coating carbon powder on the surface layer of the nodular cast iron part according to the thickness of the carbon powder layer calculated in the step 1), then placing the nodular cast iron part coated with the carbon powder in a crucible, and then covering the crucible with a cover; after the furnace temperature reaches the set temperature, placing the crucible in the furnace; after the heat preservation time reaches the austenitizing heat preservation time calculated in the step 1), quickly placing the nodular cast iron workpiece into water-based quenching liquid at the temperature of 60-80 ℃ for quick cooling for 2-6 seconds;

step 3), low-temperature isothermal treatment control;

and quickly taking out the quenched nodular cast iron part, and placing the nodular cast iron part at the temperature of 160-220 ℃ for heat preservation treatment for 5-10 hours.

Step 4), air cooling control; and taking out the nodular cast iron product, suspending and air-cooling to room temperature.

Technical Field

The invention relates to a novel quenching-partitioning-isothermal heat treatment process for gradient of surface microstructure of a nodular cast iron product and improvement of wear resistance, and belongs to the technical field of hot processing of cast iron products.

Background

Wear is one of the main causes of failure of mechanical parts, and statistically about 75% of machine parts are scrapped due to wear. The material consumption of China is more than 100 million tons each year due to abrasion, and billions of yuan of economic loss is caused by the shutdown and maintenance cost. Research shows that about 1/3-1/2 of energy consumption in the world is on various forms of frictional wear, and the loss caused by the energy consumption is up to $ 1000 billion. Therefore, the improvement of the wear resistance of the material is a technical research and development with great social and economic significance.

The nodular cast iron has good comprehensive performance, draws the wide attention of scholars at home and abroad, and becomes one of the materials with the most development potential in the 21 st century. The wear-resistant steel has the advantages of high strength, high toughness and the like, and has the specific self-lubricating and strain strengthening performances, so that the wear-resistant steel has strong wear resistance, is widely applied to preparation of various wear parts, and has obvious economic benefit.

With the increasing application of high-performance nodular cast iron as a substitute of forged steel and even aluminum alloy in the mechanical manufacturing and defense industry, how to maintain higher frictional wear resistance under heavy load and impact environment during service has been increasingly emphasized.

The surface of the ductile iron product is the area which is firstly worn, and is also the area with the largest deformation and the largest stress, if the external force borne by the product in service is fully utilized to generate deformation, and corresponding phase transformation (such as deformation-induced martensite phase transformation) is induced or promoted through the deformation, a multi-phase structure organization is formed on the surface, so that a coordinated strengthening and toughening effect is formed on the surface, and the surface resistance of the ductile iron product is improved. Meanwhile, the properties, shapes, quantity and distribution of the multi-phase structure of the subsurface and the core of the workpiece are adjusted, so that the workpiece always keeps certain hardness and toughness, and the aim of integrally improving the wear resistance is fulfilled.

Disclosure of Invention

The invention aims to solve the existing problems and provides a novel quenching-partitioning-isothermal heat treatment process for graduating the surface microstructure of a nodular cast iron product and improving the wear resistance.

The invention aims to realize a novel quenching-distribution-isothermal heat treatment process for graduating the surface microstructure of a nodular cast iron product and improving the wear resistance, which is characterized by comprising the following steps of:

step 1), austenitizing control; the austenitizing process comprises two parts of control, namely parameter control of the austenitizing process and control of the concentration and thickness of carbon powder on the surface layer of the nodular cast iron product;

i, controlling parameters of an austenitizing process as follows:

austenitizing temperature (T)_AAnd deg.C) is formulated according to equation (1):

in the formula: t is_AAustenitizing temperature in degrees celsius; % C is the mass fraction of carbon element; % Ni is the mass fraction of nickel element; % Si is the mass fraction of silicon element; % V is the mass fraction of vanadium; % Mo is the mass fraction of molybdenum element; % W is the mass fraction of tungsten element; % Mn is the mass fraction of manganese element; % Cr is the mass fraction of chromium element; % Cu is the mass fraction of copper element; % P is the mass fraction of phosphorus element; % Al is the mass fraction of aluminum element; % A is the mass fraction of arsenic; % Ti is the mass fraction of titanium element;

austenitizing Heat preservation time (t)_AMinutes) is formulated according to equation (2):

t_A＝3H+18 (2)

in the formula: h is the thickness of the nodular cast iron part, and the unit is millimeter; t is t_AAustenitizing holding time in minutes;

II, controlling parameters of the austenitized carbon powder as follows:

the surface layer of the nodular cast iron product is coated with carbon powder, wherein the concentration of the carbon powder is more than or equal to 98.0 percent, and the particle size of the carbon powder is less than or equal to 1 × 10^-6Millimeter;

coating carbon powder on the surface layer of the nodular cast iron part, and calculating the thickness d of the carbon powder layer according to a formula (3):

wherein d is the thickness of the carbon powder layer and the unit is meter, Q is the diffusion activation energy of carbon atoms in austenite, 1.34 × 10⁵Coke/mole ratioEr (Chinese character of 'Er')(ii) a R is a gas constant of 8.314J/(mol)Er (Chinese character of 'Er')On); t is absolute temperature, and the unit is on; d is a diffusion coefficient; wherein D can be determined by calculation of formula (4);

in the formula: c_s＝0.02；C＝0.85C₀；C₀Is the original mass fraction of carbon element in steel,%; t is austenitizing heat preservation time for minutes;

step 2), setting the temperature of a heating furnace according to the austenitizing temperature calculated in the step 1), coating carbon powder on the surface layer of the nodular cast iron part according to the thickness of the carbon powder layer calculated in the step 1), then placing the nodular cast iron part coated with the carbon powder in a crucible, and then covering the crucible with a cover; after the furnace temperature reaches the set temperature, placing the crucible in the furnace; after the heat preservation time reaches the austenitizing heat preservation time calculated in the step 1), quickly placing the nodular cast iron workpiece into water-based quenching liquid at the temperature of 60-80 ℃ for quick cooling for 2-6 seconds;

step 3), low-temperature isothermal treatment control;

and quickly taking out the quenched nodular cast iron part, and placing the nodular cast iron part at the temperature of 160-220 ℃ for heat preservation treatment for 5-10 hours.

Step 4), air cooling control; and taking out the nodular cast iron product, suspending and air-cooling to room temperature.

The method is advanced and scientific, and the novel quenching-distribution-isothermal heat treatment process for the surface microstructure gradient and the wear resistance improvement of the nodular cast iron product, provided by the invention, has the important influence factor on the control of the carbon atom content on the surface and inside of the nodular cast iron product. Carbon is an element for stabilizing austenite, the carbon content in the austenite is different, and the C curve positions are different. With the reduction of the austenite carbon content, the incubation period of the super-cooled austenite isothermal transformation is shortened, and the C curve moves to the left. According to the invention, the carbon atom concentration from the surface layer to the core part is controlled in the austenitizing process to form the carbon-poor austenite on the surface layer of the workpiece, and the austenite contents of the subsurface and the core part are increased gradually, so that a multi-phase structure microstructure corresponding to the high wear resistance requirement is formed in each region.

Compared with the conventional nodular cast iron in industry, the nodular cast iron after the heat treatment process has the following advantages:

after the heat treatment process is carried out, the matrix of the ball-milling cast iron part is different from the industrial conventional double-phase microstructure of pearlite and ferrite and is a multi-phase microstructure consisting of upper bainite, tempered martensite, bainite ferrite and residual austenite. The strength and ductility and toughness of the article may be improved by the interaction of strengthening phases (e.g., tempered martensite and bainite ferrite) and toughening phases (e.g., retained austenite) therein.

Compared with the conventional nodular cast iron, the surface-to-core microstructure of the nodular cast iron part obtained by the invention has gradient change, wherein the content of upper bainite decreases from 50% to 5% from the surface to the inside, and the content of tempered martensite increases from 5% to 20% from the surface to the inside.

The novel heat treatment method scheme can lead the upper bainite in the multiphase microstructure to generate larger deformation in the friction process of the surface of a workpiece, thereby realizing deformation strengthening; tempered martensite and bainite ferrite have higher hardness, and can improve the friction resistance; under the action of frictional shear force, austenite generates martensite phase transformation, namely the so-called TRIP effect, so as to achieve the purpose of strain-induced plasticity. That is, the wear resistance of the nodular cast iron is effectively improved by fully utilizing the toughness of each component phase.

The invention adopts a novel quenching-distribution-isothermal heat treatment process for graduating the microstructure of the surface of a nodular cast iron workpiece and improving the wear resistance to obtain the nodular cast iron with the above tissue gradient structure and high wear resistance. The process has novelty (no setting and design of the process parameters exist at present), advancement (a multiphase structure and a performance gradient are directly formed on the cast iron workpiece from the surface to the inside) and practicability (the process is not complex and can be directly applied to actual industrial production), so that the process has wide industrial application prospect.

Drawings

FIG. 1 shows the microstructure of a Scanning Electron Microscope (SEM) of a longitudinal section (vertical friction surface) of a nodular cast iron part treated according to the invention after 200N friction wear.

Fig. 2 shows the strain martensite Scanning Electron Microscope (SEM) appearance of the longitudinal section (vertical friction surface) of the nodular cast iron part treated by the present invention after 200N friction wear.

Fig. 3 is a comparison of the wear rates of the nodular cast iron parts treated by the present invention and the conventional nodular cast iron parts under different wear load conditions.

Detailed Description

A novel quenching-distribution-isothermal heat treatment process for gradient microstructure and improvement of wear resistance of a surface of a nodular cast iron workpiece comprises the following steps:

step 1), austenitizing control; the austenitizing process comprises two parts of control, namely parameter control of the austenitizing process and control of the concentration and thickness of carbon powder on the surface layer of the nodular cast iron product;

i, controlling parameters of an austenitizing process as follows:

austenitizing temperature (T)_AAnd deg.C) is formulated according to equation (1):

in the formula: t is_AAustenitizing temperature in degrees celsius; % C is the mass fraction of carbon element; % Ni is the mass fraction of nickel element; % Si is the mass fraction of silicon element; % V is the mass fraction of vanadium; % Mo is the mass fraction of molybdenum element; % W is the mass fraction of tungsten element; % Mn is the mass fraction of manganese element; % Cr is the mass fraction of chromium element; % Cu is the mass fraction of copper element; % P is the mass fraction of phosphorus element; % Al is the mass fraction of aluminum element; % A is the mass fraction of arsenic; % Ti is the mass fraction of titanium element;

austenitizing Heat preservation time (t)_AMinutes) is formulated according to equation (2):

t_A＝3H+18 (2)

in the formula: h is the thickness of the nodular cast iron part, and the unit is millimeter; t is t_AAustenitizing holding time in minutes;

II, controlling parameters of the austenitized carbon powder as follows:

the surface layer of the nodular cast iron product is coated with carbon powder, wherein the concentration of the carbon powder is more than or equal to 98.0 percent, and the particle size of the carbon powder is less than or equal to 1 × 10^-6Millimeter;

coating carbon powder on the surface layer of the nodular cast iron part, and calculating the thickness d of the carbon powder layer according to a formula (3):

wherein d is the thickness of the carbon powder layer and the unit is meter, Q is the diffusion activation energy of carbon atoms in austenite, 1.34 × 10⁵Coke/mole ratioEr (Chinese character of 'Er')(ii) a R is a gas constant of 8.314J/(mol)Er (Chinese character of 'Er')On); t is absolute temperature, and the unit is on; d is a diffusion coefficient; wherein D can be determined by calculation of formula (4);

in the formula: c_s＝0.02；C＝0.85C₀；C₀Is the original mass fraction of carbon element in steel,%; t is austenitizing heat preservation time for minutes;

step 2), setting the temperature of a heating furnace according to the austenitizing temperature calculated in the step 1), coating carbon powder on the surface layer of the nodular cast iron part according to the thickness of the carbon powder layer calculated in the step 1), then placing the nodular cast iron part coated with the carbon powder in a crucible, and then covering the crucible with a cover; after the furnace temperature reaches the set temperature, placing the crucible in the furnace; after the heat preservation time reaches the austenitizing heat preservation time calculated in the step 1), quickly placing the nodular cast iron workpiece into water-based quenching liquid at the temperature of 60-80 ℃ for quick cooling for 2-6 seconds;

step 3), low-temperature isothermal treatment control;

and quickly taking out the quenched nodular cast iron part, and placing the nodular cast iron part at the temperature of 160-220 ℃ for heat preservation treatment for 5-10 hours.

Step 4), air cooling control; and taking out the nodular cast iron product, suspending and air-cooling to room temperature.

The method specifically comprises the following steps:

(1) firstly, calculating the austenitizing temperature and the heat preservation time corresponding to the ductile cast iron product according to a formula (1) and a formula (2).

(2) And (3) selecting proper carbon powder, and calculating the thickness of the carbon powder layer correspondingly controlled by the nodular cast iron part according to a formula (3) and a formula (4).

(3) And setting the temperature of the heating furnace according to the calculated austenitizing temperature and time, wrapping the workpiece in the crucible according to the calculated carbon powder thickness, and covering the crucible.

(4) After the furnace temperature reaches the set temperature, the crucible is placed in the furnace.

(5) And after the heat preservation time reaches the set temperature, quickly placing the workpiece into water-based quenching liquid at the temperature of 60-80 ℃ for quick cooling for 2-6 seconds.

(6) The part is rapidly placed in a furnace at the temperature of 160-220 ℃ for tempering, and the tempering time is 5-10 hours.

(7) And after the tempering is finished, taking the workpiece out of the furnace, and cooling to room temperature.

Example 1:

the method adopts a raw material with chemical elements of 3.2C-2.5Si-0.2Mn-0.25Mo-0.75Cu, the microstructure of the part matrix treated by the method is converted from pearlite and ferrite into a multi-phase structure microstructure consisting of upper bainite, tempered martensite, bainite ferrite and residual austenite, the content of the upper bainite is decreased from 50% to 5% from the surface to the inside, and the content of the tempered martensite is increased from 5% to 20% from the surface to the inside. The surface microstructure has gradient material change of upper bainite and tempered martensite, and the upper bainite is easy to generate strong deformation on the surface during the friction and wear process, as shown in figure 1.

The surface of the material is strengthened by work hardening via frictional wear at the upper bainite to promote deformation.A large amount of strain-induced martensite is formed in the bulk austenite region, as shown in FIG. 2. the material has a wear rate of 2.5 × 10 at 200N load of 2.5^-3mg/m。

Example 2:

the raw material with chemical elements of 3.6C-2.4Si-0.4Mn-0.1Mo-0.8Cu is austenitized at 900 ℃ for 90min, isothermally treated at 290 ℃ for 90min and air-cooled to room temperature, and then under the conditions of 150N load and no difference from the rest of abrasion conditions of the example 1, the abrasion rate is 2.63 × 10^-3mg/m. The wear resistance of the nodular cast iron processed by the traditional isothermal quenching process is inferior to that of the nodular cast iron processed by the process of the invention. The comparative data are shown in FIG. 3. The reason is that the strong work hardening action greatly improves the wear resistance of the material, and compared with the traditional austempered ductile iron, the novel gradient material surface containing the upper bainite structure has stronger deformability, can generate the strong work hardening action and enables the material surface to have higher wear resistance.