CN104164548B - A kind of thermal treatment process of thick and large section low-carbon low-alloy steel forge piece - Google Patents

A kind of thermal treatment process of thick and large section low-carbon low-alloy steel forge piece Download PDF

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CN104164548B
CN104164548B CN201410273812.3A CN201410273812A CN104164548B CN 104164548 B CN104164548 B CN 104164548B CN 201410273812 A CN201410273812 A CN 201410273812A CN 104164548 B CN104164548 B CN 104164548B
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CN104164548A (en
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蒋中华
王培�
李殿中
李依依
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Institute of Metal Research of CAS
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Abstract

The invention belongs to ferrous materials Field of Heat-treatment, a particularly thermal treatment process for thick and large section low-carbon low-alloy steel forge piece, when being adapted to solve big cross section low-carbon low-alloy steel forge piece thermal treatment large because of wall thickness, quench and thoroughly do not produce the low and problem of instability of low-temperature impact toughness in heart portion.The invention is characterized in traditional quenching, the intermediate heat treatment in the anneal of a below Ac1 50 DEG C ~ 90 DEG C of temperature and an Ac3 ± 25 DEG C temperature range is added between tempering heat treatment, correspondingly adjust the tempering temperature of traditional technology simultaneously, thus be reached through the content that intermediate heat treatment regulates martensite/austenite island structure in thick and large section heart portion granular bainite microstructure (being called for short M-A island), size, carbon concentration, the object of hardness and distributing position, realize the matched well with matrix, thick and large section steel forge piece core structure is made to have excellent obdurability further by suitable tempering heat treatment control granular bainite matrix and M-A island in the evolution of drawing process.

Description

A kind of thermal treatment process of thick and large section low-carbon low-alloy steel forge piece
Technical field
The invention belongs to ferrous materials Field of Heat-treatment, particularly the thermal treatment process of a kind of thick and large section low-carbon low-alloy steel forge piece such as (as: Cr ?Mo steel).
Background technology
Low-carbon and low-alloy Cr ?Mo steel owing to having better obdurability and excellent anti-hydrogen embrittlement performance, widespread use and large-scale hydrogenator, nuclear power cylinder body and some the strength of materials, low-temperature flexibility and resistance to elevated temperatures are had certain requirements on parts.Under traditional industry application conditions, low-carbon and low-alloy Cr ?the standard thermal treatment technique of Mo steel forge piece be quenching+tempering.Obtain lower bainite or martensitic stucture after quenching, after then carrying out the temper of proper temperature, the intensity of material and plasticity and toughness can reach matched well.
But along with the continuous expansion of hydrogenation unit scale and the continuous increase of nuclear power power, low-carbon and low-alloy Cr ?the size of Mo steel forge piece and wall thickness significantly increase, quench cooled scarce capacity in heat treatment process is manufactured in reality, although have employed the type of cooling that sprays water with high pressure when some heavy froging is quenched to improve quench cooling rate, but its heart portion is still difficult to through hardening, easily cause forging gross distortion even to ftracture simultaneously.Therefore, thick and large section (referring generally to section thickness is 100 ~ 600mm) low-carbon and low-alloy CrMo steel forge piece after quenching heart portion easily obtains, based on the middle temperature transformation product of the poor granular bainite of toughness, having had a strong impact on the low-temperature impact toughness in large-scale workpiece heart portion.
In order to improve the heart portion performance of thick and large section low-carbon and low-alloy CrMo steel forge piece, also some bibliographical informations are had, adopt sub-temperature critical heat treatment process to improve the low-temperature impact toughness of material, due to sub-critical temperature austenitizing, make in tissue higher containing undissolved ferrite content, not only greatly reduce the intensity of material, and significantly reduce the high temperature endurance performance of material, be not suitable for for there being the part forge piece of particular requirement to material at high temperature performance using.
Therefore, under the condition not relying on cooling rate, how realized the good combination of thick and large section low-alloy CrMo steel and other low-carbon and low-alloy steel core portion's intensity and plasticity and toughness by brief heat treating technique, the manufacturing for thick and large section low-alloy CrMo steel and other low-carbon low-alloy steel forge pieces is significant.
Summary of the invention
The object of the present invention is to provide a kind of thermal treatment process of thick and large section low-carbon low-alloy steel forge piece, under the prerequisite as far as possible do not increased or increase production cost on a small quantity, do not change casting, forging alloying constituent, by means of only improvement thermal treatment process, adjust each phase character in material structure, regulate the total amount of each phase, size and distribution, improve the thick and large section low-carbon low-alloy steel forge piece core structure such as (as: Cr ?Mo steel), and then the intensity of heavy casting and forging and plasticity and toughness are all effectively improved.
Technical scheme of the present invention is:
A thermal treatment process for thick and large section low-carbon low-alloy steel forge piece, this technique comprises the following steps:
(1) thick and large section low-carbon low-alloy steel forge piece is carried out quench treatment within the scope of more than Ac3 40 DEG C ~ 100 DEG C, be incubated and often increase the calculating in 0.5 ~ 1 hour of 25mm holding time by wall thickness;
(2) after having quenched, carry out the anneal of below Ac1 50 DEG C ~ 90 DEG C of temperature, be incubated and often increase the calculating in 0.5 ~ 2 hour of 25mm holding time by wall thickness, cool with air cooling or the cold mode of stove after insulation terminates;
(3) after having annealed, carry out the intermediate heat treatment in DEG C temperature range of Ac3 ± 25, be incubated and often increase the calculating in 0.5 ~ 1 hour of 25mm holding time by wall thickness;
(4) after intermediate heat treatment, carry out the temper in below Ac1 60 DEG C ~ 110 DEG C temperature ranges, be incubated and often increase the calculating in 1 ~ 2 hour of 25mm holding time by wall thickness.
The thermal treatment process of described thick and large section low-carbon low-alloy steel forge piece, step 1) quenching heat treatment, step 3) intermediate heat treatment and step 4) tempering heat treatment, do not causing workpiece to ftracture or under gross distortion condition, adopting speed of cooling to be as quickly as possible cooled to room temperature after insulation terminates.
The thermal treatment process of described thick and large section low-carbon low-alloy steel forge piece, step 3) intermediate heat treatment, austenitizing temperature will control in DEG C temperature range of Ac3 ± 25, and when when below Ac3 austenitizing, undissolved ferrite content should be controlled within 10wt%.
The thermal treatment process of described thick and large section low-carbon low-alloy steel forge piece, step 4) tempering heat treatment that adopts, fully take into account M ?A island situation of decomposing the obdurability of material affect, avoid separating out the thick carbide distributed along crystal boundary.
The thermal treatment process of described thick and large section low-carbon low-alloy steel forge piece, low-carbon low-alloy steel be alloying element content be no more than 5wt.% Cr ?Mo system steel; Or low-carbon low-alloy steel is the low-alloy bainite steel that alloying element content is no more than 5wt.%: NiCr system, MnMoNi system, SiMn system, SiMnMo system, CrMnSi system, CrMnMo system or CrNiMo system.By weight percentage, low-carbon low-alloy steel alloying element mainly comprises: C≤0.30%, Mn≤1.5%, Cr≤4%, Mo≤1.5%, V≤0.5% and trace alloying element Nb, Ti, B more than one, the massfraction summation of alloying element is no more than 5%.
The thermal treatment process of described thick and large section low-carbon low-alloy steel forge piece, the wall thickness of thick and large section low-carbon low-alloy steel forge piece is at 100 ~ 600mm.
Design philosophy of the present invention and principle as follows:
The present invention and existing technique improve thick and large section low-carbon low-alloy steel (as: CrMo steel etc.) forge piece heart portion performance thinking by changing cooling rate has remarkable different.Main thought of the present invention is under the condition not changing cooling rate, between conventional modulated heat-treatment quenching and tempering heat treatment, by adding intermediate treatment, change granular bainite in continuous coo1ing kinetic transformation, control the formability of thick and large section low-carbon low-alloy steel (as: CrMo steel etc.) forge piece heart portion granular bainite, the character on martensite/austenite island particularly in granular bainite (M ?A island), total amount, pattern, size, distribution, and control the evolution in drawing process of granular bainite matrix and island by follow-up suitable adjustment tempering heat treatment system, thus the tough matching optimization object reached low-carbon and low-alloy nodular bainite steel.Finally, make to adopt heat treatment condition of the present invention to obtain, than prior heat treatment process, there is better obdurability coupling.Concrete principle is as follows:
1, the present invention adopt Ac3 ± 25 DEG C temperature range in intermediate heat treatment, require that austenitizing undissolved ferrite content controls within 10wt%, be cooled to room temperature with certain speed of cooling after insulation, object makes the microstructure of material that certain change occur.Specifically namely, changed the continuous cooling transformation (namely changing the position of CCT curve) of low-carbon low-alloy steel (as: Cr ?Mo steel etc.) by intermediate heat treatment, particularly significantly impact is produced on granular bainite transition kinetics.Intermediate heat treatment causes CCT curve to move to left, reduce the formability of granular bainite, make phase transformation at a higher temperature, carry out with speed more slowly, in granular bainite M ?in the tissue of A island rich carbon degree reduce, M ?A island and matrix relative hardness difference reduces, simultaneously M ?the total amount on A island and size all necessarily reduced, and it is more even to distribute.Thus can be implemented in and do not reduce the strength of materials or under slightly falling low intensive condition, increase substantially the impelling strength of material, can be good at the obdurability optimizing material.
2, simultaneously, the present invention adds thermal treatment of annealing together before intermediate heat treatment, main purpose eliminates the rate of heating that may bring using nonequilibrium state tissue (quenching structure) as intermediate heat treatment original structure to the impact of structural transformation, and heavy casting and forging can be avoided to be out of shape in intermediate heat treatment process, ftracture and Structure Inheritance.
3, the present invention devises tempering process after intermediate heat treatment, the unrelieved stress that material produces when quenching can not only be eliminated, and can to M in granular bainite microstructure ?A island the evolution of drawing process and bainite matrix softening in play an important role, and then the problem that respectively matches in better optimizing tissue, finally make material have good comprehensive mechanical property.
The technical superiority of the present invention and existing process ration:
Can find out based on above-mentioned principle, compared with prior art there is following advantage in present invention process: the intermediate heat treatment in (1) the present invention mainly reaches by regulating granular bainite two to match the comprehensive mechanical property improving material, and traditional critical zone thermal treatment austenitizing temperature relatively low (generally between more than Ac1 30 ~ 50 DEG C), mainly utilize the amount controlling undissolved ferrite and distribution, form optimizes the mechanical property of material, therefore employing critical zone thermal treatment improves the impelling strength of material often by sacrificing some strength, low-carbon low-alloy steel (as: CrMo steel etc.) forge piece can not be met to the requirement of heat resistance.(2) material of the present invention before intermediate heat treatment is equilibrium state tissue, is conducive to obtaining organizing more uniformly after intermediate heat treatment; And general critical zone thermal treatment makes an addition between quenching-and-tempering process, namely using nonequilibrium state tissue (quenching structure and quenching are without abundant tempered structure) as critical heat process, easily make workpiece (especially for heavy casting and forging) be out of shape during this thermal treatment process, ftracture, simultaneously nonequilibrium state organizes heating again to inherit to heat for the first time thick tissue, the material formed to become complexity in heat-processed structural transformation, for heavy casting and forging, the instability of performance may be caused due to the inequality of rate of heating.
Advantage of the present invention and beneficial effect as follows:
1, the present invention is under the chemical composition condition not changing large pressurized vessel low-carbon low-alloy steel (as: Cr ?Mo steel etc.), by means of only the optimization of Heat Treatment Process to material, the character of each phase in adjustment Fine Texture of Material, regulate the total amount of each phase, size and distribution, and by optimizing tempering process to each evolution in follow-up drawing process, and then improve the obdurability coupling of such steel.
2, the present invention is applied widely, be not only applicable to allly be applied to large pressurized vessel low-carbon and low-alloy Cr ?Mo system steel, also can be no more than the low-carbon and low-alloy bainitic steels such as the NiCr system of 5wt.%, MnMoNi system, SiMn system, SiMnMo system, CrMnSi system, CrMnMo system or CrNiMo system for alloying element content, conventional alloy designations can for but be not limited to 2.25Cr1Mo ?0.25V, G18CrMo2 ?6, SA508 ?3,15CrMoVA, 18Cr 2ni 4wA, 12CrNi3,35SiMnMo, 15SiMnVTi or 25SiMoVB etc.
Accompanying drawing explanation
Fig. 1 is thermal treatment process schematic diagram of the present invention.
Fig. 2 is prior heat treatment process schematic diagram.
Embodiment
In order to improve the thick and large section low-carbon low-alloy steel forge piece heart portion obdurability such as (as: Cr ?Mo steel) coupling, the present invention proposes and increase by one anneal and intermediate heat treatment after the quenching technology of traditional quenching+tempering (i.e. Tempering and Quenching), suitably adjust the temperature of tempering heat treatment simultaneously, thus reach under the limited prerequisite of cooling power, make material obtain the scheme of desirable obdurability coupling by changing the content of each composition phase in granular bainite microstructure, size and distribution.Concrete implementation step is as follows:
(1) dilatometry is first used to measure Ac1 and the Ac3 point temperature intending the forge piece material of heat-treating, to formulate subsequent heat treatment temperature range;
(2) thick and large section low-carbon low-alloy steel (as: CrMo steel etc.) forge piece is carried out quench treatment within the scope of more than Ac3 40 DEG C ~ 100 DEG C, be incubated and often increase the calculating in 0.5 ~ 1 hour of 25mm holding time by wall thickness; After insulation terminates, ftracture not causing workpiece or adopt speed of cooling to be as quickly as possible cooled to room temperature under gross distortion condition;
(3) after having quenched, carry out the anneal of below Ac1 50 DEG C ~ 90 DEG C of temperature, be incubated and often increase the calculating in 0.5 ~ 2 hour of 25mm holding time by wall thickness, after insulation terminates, be cooled to room temperature with air cooling or the cold mode of stove;
(4) after having annealed, carry out the intermediate heat treatment in DEG C temperature range of Ac3 ± 25, be incubated and often increase the calculating in 0.5 ~ 1 hour of 25mm holding time by wall thickness; After insulation terminates, ftracture not causing workpiece or adopt speed of cooling to be as quickly as possible cooled to room temperature under gross distortion condition;
(5) after intermediate heat treatment, carry out the temper in below Ac1 60 DEG C ~ 110 DEG C temperature ranges, be incubated and often increase the calculating in 1 ~ 2 hour of 25mm holding time by wall thickness; After insulation terminates, ftracture not causing workpiece or adopt speed of cooling to be as quickly as possible cooled to room temperature under gross distortion condition.
In the present invention, described " insulation often increases 25mm holding time by wall thickness ", its implication is:
Thick and large section low-carbon low-alloy steel (as: CrMo steel etc.) forge piece heart portion obdurability matching effect is significantly improved in order to well embody thermal treatment process of the present invention.In follow-up specific embodiment, the prior heat treatment process that the present invention enumerates, is used for superiority of the present invention is described, but prior heat treatment process do not mean that and protect restriction by the present invention.
In order to proved invention has wider range of application, and considering the feasibility of actually operating, the present invention selects a kind of typical thick and large section low-carbon and low-alloy CrMo steel forgings (hydrogenator 2.25Cr1Mo0.25V steel thick plate) and one typical thick and large section low-carbon and low-alloy CrMo steel casting (nuclear power low pressure G18CrMo2 ?6 steel cylinder bodies) as case study on implementation respectively.Meanwhile, the present invention also can be used for MnMoNi system low-carbon and low-alloy steel forgings, is described in an embodiment for nuclear power SA508 ?3 steel thick and large section pressurized vessel.
Embodiment 1
In the present embodiment, large-scale hydrogenator 2.25Cr ?1Mo ?the thermal treatment of 0.25V steel thick plate, detailed process is as follows:
(1) first chemical composition analysis is carried out to slab sampling, test result is 0.15C (weight percent, down together), 0.05Si, 2.46Cr, 1.01Mo, 0.28V, 0.15Ni, 0.006P, 0.002S, Fe surplus, result meet relevant criterion to 2.25Cr ?1Mo ?the requirement of 0.25V steel chemical composition, and belong to the scope of application of the present invention.Rolling slab is got the pole of φ 3x10mm, and Ac3, Ac1 of using methods of thermal expansion to record material are respectively 886 DEG C, 776 DEG C.
(2) after heat treatment after forging, be the slab that 400mm is thick to wall thickness, carry out the austenitizing process of 980 DEG C × 8h, then carry out spray quenching process;
(3) after having quenched, slab is done anneal at 725 DEG C, annealing soaking time is 16 hours, adopts air cooling mode to be cooled to room temperature;
(4), after anneal, slab is done respectively the intermediate heat treatment (technique one, technique two austenitizing temperature are respectively 905 DEG C, 870 DEG C) of the present embodiment technique, thermal treatment soaking time is 8h, and after coming out of the stove, water-spraying control is to room temperature.
(5) as shown in Figure 1, thermal treatment process schematic diagram of the present invention.The coupon of step (4) distinguishes suitable tempering heat treatment, and technique one, technique two tempering temperature of the present embodiment are respectively 690 DEG C and 670 DEG C, and thermal treatment soaking time is 16h, and after coming out of the stove, water-spraying control is to room temperature.
(6) as shown in Figure 2, prior heat treatment process schematic diagram.In order to contrast with the process of the present embodiment thermal treatment process, other one piece of same material, 2.25Cr1Mo0.25V steel thick plate with size, prior heat treatment process is used to process, concrete thermal treatment process is 940 DEG C × 8h quench treatment+700 DEG C × 16h temper, and the Quench and temper type of cooling is water-spraying control.
(7) from prior heat treatment process, Optimization Technology one and Optimization Technology two, the test block of same size is got in slab centre after three kinds of art breading, be processed into tension specimen and summer than V-type impact specimen, test normal, drawing by high temperature mechanical property respectively, and low temperature impact properties, test result is as shown in table 1.From experimental results, the present embodiment adopts technique one and technique two to be all better than prior heat treatment process.
Table 1: in embodiment 1 2.25Cr ?1Mo ?0.25V steel thick plate adopt the present embodiment process optimization and traditional technology mechanical property to contrast
(8) table 2 gives and uses the present embodiment art breading and traditional technology process 2.25Cr1Mo0.25V steel thick plate centre microstructure feature, contrast can be found out, after using the present embodiment art breading, M in granular bainite ?A island quantity and size all decline to some extent than traditional technology, simultaneously M ?A island be distributed in intracrystalline more, and hardness declines to some extent.M ?the change of these parameters of A island meet the principle of design of the present embodiment, be the reason that material impact toughness improves.
Table 2: in embodiment 1 2.25Cr ?1Mo ?0.25V steel thick plate adopt microstructure characteristic after the present embodiment technique and traditional technology thermal treatment
Embodiment 2
In the present embodiment, nuclear power with G18CrMo2 ?6 steel low pressure (LP) cylinder body heat process, detailed process is as follows:
(1) the present embodiment is massive casting, material be G18CrMo2 ?6 steel, thickest 500mm is thick.First detecting the present embodiment material main chemical compositions is 0.15C, 0.61Cr, 0.61Mo, 0.75Mn, 0.75Mn, 0.46Ni, Fe surplus, meets the scope of application of the present invention.Transformation temperature Ac1, the Ac3 of the material utilizing thermal expansion to survey are respectively: 760 DEG C, 880 DEG C.
(2) identical with embodiment 1, first the present embodiment also quenches to foundry goods and thermal treatment of annealing, but distinguishes with embodiment 1 and be, the present embodiment quenching austenitizing temperature is 920 DEG C, insulation 20 hours, and blowing is chilled to room temperature; Annealing temperature is 670 DEG C, insulation 40h, and the cold mode of stove is cooled to room temperature.
(3) after anneal, the present embodiment carries out intermediate heat treatment to relevant foundry goods, and treatment temp adopts two kinds of differing tempss, that is: technique one, 890 DEG C of normalizings; Technique two, 855 DEG C of normalizings).Soaking time is 15h, and after insulation terminates, blowing is chilled to room temperature.
(4) after normalizing treatment, the present embodiment carries out temper to relevant foundry goods, and the tempering temperature of technique one is 700 DEG C, and the tempering temperature of technique two is 650 DEG C.Soaking time is 30h, and after insulation terminates, blowing is chilled to room temperature.
(5) in order to contrast with the process of the present embodiment thermal treatment process, to identical material, the foundry goods of same size, carries out prior heat treatment process process (920 DEG C × 20h normalizing treatment, blowing is chilled to room temperature+680 DEG C × 30h temper, and blowing is chilled to room temperature).
(6) from prior heat treatment process, Optimization Technology one and Optimization Technology two, the test block of same size is got in centre, foundry goods thickest place after three kinds of art breading, be processed into tension specimen and summer than V-type impact specimen, test normal, drawing by high temperature mechanical property respectively, and low temperature impact properties, test result is as shown in table 3.From experimental results, the present embodiment adopts technique one and technique two to be all better than prior heat treatment process.
Table 3:G18CrMo2 ?6 steel adopt the present embodiment process optimization and traditional technology mechanical property to contrast
Embodiment 3
In the present embodiment, nuclear power large pressurized vessel steel heat treatment, detailed process is as follows:
(1) the present embodiment is heavy froging, material be SA508 ?3 trades mark Mn ?Mo ?Ni steel, maximum heavy wall is 350mm.The main chemical compositions of test material is 0.23C, 0.18Cr, 0.48Mo, 1.48Mn, 0.75Mn, 0.8Ni, 0.22Si, Fe surplus, wherein impurity element scope control is in the scope that standard specifies, transformation temperature Ac1, the Ac3 of the material utilizing thermal expansion to survey are respectively: 714 DEG C, 809 DEG C.
(2) identical with embodiment 1, first the present embodiment also quenches to foundry goods and thermal treatment of annealing, but distinguishes with embodiment 1 and be, the present embodiment quenching austenitizing temperature is 860 DEG C, insulation 12 hours, and oil quenching is to room temperature; Annealing temperature is 660 DEG C, insulation 24h, and the cold mode of stove is cooled to room temperature.
(3) after anneal, the present embodiment carries out intermediate heat treatment to relevant heavy froging, and treatment temp adopts two kinds of differing tempss, that is: technique one, 815 DEG C of normalizings; Technique two, 795 DEG C of normalizings.Soaking time is 15h, and after insulation terminates, air cooling is to room temperature.
(4) after intermediate heat treatment, the present embodiment carries out temper to relevant forging, and the tempering temperature of technique one is 650 DEG C, and the tempering temperature of technique two is 640 DEG C.Soaking time is 30h, and after insulation terminates, air cooling is to room temperature.
(5) in order to contrast with the process of the present embodiment thermal treatment process, to identical material, the forging of same size, carries out prior heat treatment process process (860 DEG C × 15h normalizing treatment, air cooling is to room temperature+660 DEG C × 28h temper, and air cooling is to room temperature).
(6) from prior heat treatment process, Optimization Technology one and Optimization Technology two, the test block of same size is got in centre, foundry goods thickest place after three kinds of art breading, be processed into tension specimen and summer than V-type impact specimen, test tensile mechanical properties respectively, and low temperature impact properties, test result is as shown in table 4.From experimental results, the present embodiment adopts technique one and technique two to be all better than prior heat treatment process.
Table 4:SA508 ?3 steel adopt the present embodiment process optimization and traditional technology mechanical property to contrast
Embodiment result shows, the present invention is in traditional quenching, the intermediate heat treatment in the anneal of a below Ac1 50 DEG C ~ 90 DEG C of temperature and an Ac3 ± 25 DEG C temperature range is added between tempering heat treatment, correspondingly adjust the tempering temperature of traditional technology simultaneously, thus be reached through the content that intermediate heat treatment regulates martensite/austenite island structure in thick and large section heart portion granular bainite microstructure (be called for short M ?A island), size, carbon concentration, the object of hardness and distributing position, realize the matched well with matrix, further by suitable tempering heat treatment control granular bainite matrix and M ?A island make thick and large section steel forge piece core structure have excellent obdurability in the evolution of drawing process.Thus when can solve the thermal treatment large because of wall thickness of big cross section low-carbon low-alloy steel forge piece, quench and thoroughly do not produce the low and problem of instability of low-temperature impact toughness in heart portion.

Claims (4)

1. a thermal treatment process for thick and large section low-carbon low-alloy steel forge piece, is characterized in that, this technique comprises the following steps:
(1) thick and large section low-carbon low-alloy steel forge piece is carried out quench treatment within the scope of more than Ac3 40 DEG C ~ 100 DEG C, be incubated and often increase the calculating in 0.5 ~ 1 hour of 25mm holding time by wall thickness;
(2) after having quenched, carry out the anneal of below Ac1 50 DEG C ~ 90 DEG C of temperature, be incubated and often increase the calculating in 0.5 ~ 2 hour of 25mm holding time by wall thickness, cool with air cooling or the cold mode of stove after insulation terminates;
(3) after having annealed, carry out the intermediate heat treatment in DEG C temperature range of Ac3 ± 25, be incubated and often increase the calculating in 0.5 ~ 1 hour of 25mm holding time by wall thickness;
(4) after intermediate heat treatment, carry out the temper in below Ac1 60 DEG C ~ 110 DEG C temperature ranges, be incubated and often increase the calculating in 1 ~ 2 hour of 25mm holding time by wall thickness;
The tempering heat treatment of the quenching heat treatment of step 1), the intermediate heat treatment of step 3) and step 4), is not causing workpiece to ftracture or under gross distortion condition, is adopting speed of cooling to be as quickly as possible cooled to room temperature after insulation terminates;
The intermediate heat treatment of step 3), austenitizing temperature will control in DEG C temperature range of Ac3 ± 25, and when when below Ac3 austenitizing, undissolved ferrite content should be controlled within 10wt%.
2., by the thermal treatment process of thick and large section low-carbon low-alloy steel forge piece according to claim 1, it is characterized in that, low-carbon low-alloy steel is the Cr-Mo system steel that alloying element content is no more than 5wt.%; Or low-carbon low-alloy steel is the low-alloy bainite steel that alloying element content is no more than 5wt.%: NiCr system, MnMoNi system, SiMn system, SiMnMo system, CrMnSi system, CrMnMo system or CrNiMo system.
3. by the thermal treatment process of thick and large section low-carbon low-alloy steel forge piece according to claim 2, it is characterized in that, by weight percentage, low-carbon low-alloy steel alloying element mainly comprises: C≤0.30%, Mn≤1.5%, Cr≤4%, Mo≤1.5%, V≤0.5% and trace alloying element Nb, Ti, B more than one, the massfraction summation of alloying element is no more than 5%.
4., by the thermal treatment process of thick and large section low-carbon low-alloy steel forge piece according to claim 1, it is characterized in that: the wall thickness of thick and large section low-carbon low-alloy steel forge piece is at 100 ~ 600mm.
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