Processing method for preventing decarburization of steel pipe

Abstract

According to the machining method for preventing decarburization of the steel pipes, an Rx gas generation furnace communicates with a steel pipe calcining furnace, the first-time introduction amount of Rx gas is determined according to the volume of the steel pipe calcining furnace and the number of the steel pipes, and the proportion of all components in the Rx gas is accurately controlled, so that the steel pipes can be fully subjected to carburization reaction; and then the Rx gas and the N are supplemented again, the introduction amount of the N gas is one time of the volume of the Rx gas introduced again, and the introduction of the N can reduce the reaction activity, reduce the carbon potential, prevent the surface carbon concentration in the secondary carburizing stage from exceeding the limit, force the carbon atoms to diffuse from the surface layer of the steel pipe to the center part, and prevent the brittle carbide from being separated out in a net shape.

Description

Technical Field

[0001] This invention belongs to the field of steel pipe processing technology, and specifically relates to a processing method for preventing decarburization of steel pipes. Background Technology

[0002] In the steel pipe processing, heat treatment is required to improve the toughness and surface hardness of the steel pipe. Currently, surface decarburization is a common problem in the high-temperature heat treatment and carburizing process of steel pipes. Decarburization mechanism: Under high temperature conditions, the furnace temperature is higher than 800℃. Oxidizing components such as residual O2 and H2O in the furnace react with carbon elements on the surface of the steel pipe to generate CO or CO2 gas, which escapes, resulting in a 20-30% reduction in surface carbon content. The hardness of the decarburized layer decreases by 30-50%, which reduces the fatigue strength of the steel pipe by 20-40%, thereby affecting the service life of the products produced, such as bearings, camshafts and other key load-bearing components.

[0003] Existing improvement methods, such as inert gas protection, utilize pure N2 for supplementation, but these are costly and cannot actively replenish carbon, only delaying decarbonization.

[0004] While the graphene coating can block oxidation, it is prone to peeling off at high temperatures and can contaminate the furnace.

[0005] Traditional carburizing methods suffer from large fluctuations in CO gas composition and low precision in carbon potential control, resulting in uneven carburized layers.

[0006] Therefore, a low-cost, high-stability decarburization prevention process is needed to achieve precise control of RX gas composition; dynamic balance of carbon potential in the two stages of carburizing and N2 diffusion; and a total process time of less than or equal to 6 hours to avoid coarsening of steel pipe surface grains. Summary of the Invention

[0007] The purpose of this invention is to provide a processing method for preventing decarburization of steel pipes. An Rx gas generator is connected to a steel pipe calcining furnace. The initial Rx gas injection rate is determined based on the volume of the calcining furnace and the number of steel pipes. The proportions of each component in the Rx gas are precisely controlled to ensure the steel pipe can fully undergo the carburizing reaction. Subsequently, Rx gas and N2 are injected again, with the N2 injection rate being twice the volume of the previously injected Rx gas. The injection of N2 reduces the reactivity and carbon potential, preventing the surface carbon concentration from exceeding the limit during the secondary carburizing stage. It also forces carbon atoms to diffuse from the surface of the steel pipe towards the core, preventing the precipitation of brittle carbides in a network pattern.

[0008] This invention is achieved through the following technical solution: A processing method for preventing decarburization of steel pipes includes the following steps: S1: Connect the Rx gas generator to the steel tube calcining furnace via a pipeline; S2: Natural gas is introduced into the Rx gas generator to produce RX gas for the first time; S3: RX gas is introduced into the steel pipe calcining furnace and reacts with the steel pipe at 900–950℃ to undergo a carburizing reaction; S4: Natural gas is introduced into the Rx gas generator again to produce RX gas, and RX gas is introduced into the steel pipe calcining furnace again, while N2 is introduced at the same time to smooth out the carbon concentration gradient on the steel pipe surface. S5: After introducing N2, control the temperature of the calcining furnace at 870℃-890℃ and shut it down after 0.5-1 hour.

[0009] Preferably, in step S1, the pressure inside the steel pipe calcining furnace is maintained at 30–35 Pa, and the oxygen content is ≤3 vol.

[0010] Preferably, in step S1, the cavity volume of the steel pipe calcining furnace is 45m³, which can stack and accommodate 180-200 steel pipes.

[0011] Preferably, the steel pipes are stacked in three layers in the steel pipe calcining furnace, with the vertical spacing between adjacent steel pipes in the same layer being 1.8cm-2cm.

[0012] Preferably, in step S2, the Rx gas generator first produces 135m³-145m³ of Rx gas.

[0013] Preferably, in step S2, the RX gas is an endothermic atmosphere, and its composition by volume ratio is Co: 19-21%, H2: 38-42%, Co2: 0.25-0.45%, with the remainder being N2.

[0014] Preferably, in step S3, the carburizing reaction time between the RX gas and the steel pipe is 4.5-5 hours.

[0015] Preferably, in step S4, the Rx gas generated again by the Rx gas generator is 15 m³.

[0016] Preferably, in step S4, the volume of N2 introduced into the steel pipe calcining furnace is twice the volume of Rx gas introduced again, and the N2 introduction amount is 30 m³.

[0017] Preferably, the total calcination time of the steel pipe in the steel pipe calcination furnace is less than or equal to 6 hours.

[0018] Compared with the prior art, the present invention has the following advantages and beneficial effects: 1) In this invention, the Rx gas generator is connected to the steel pipe calcining furnace. The initial amount of Rx gas introduced is determined according to the volume of the steel pipe calcining furnace and the number of steel pipes. The proportion of each component in the Rx gas is precisely controlled so that the steel pipe can fully undergo the carburizing reaction. Then, Rx gas and N2 are introduced again. The amount of N2 gas introduced is twice the volume of Rx gas introduced again. The introduction of N2 can reduce the reaction activity, reduce the carbon potential, avoid the surface carbon concentration from exceeding the limit in the secondary carburizing stage, and force carbon atoms to diffuse from the surface of the steel pipe to the core, preventing brittle carbides from precipitating in a network.

[0019] 2) In this invention, the carburizing reaction time of the steel pipe in the steel pipe calcining furnace is 4.5-5 hours, and the total calcination time of the steel pipe in the steel pipe calcining furnace is less than or equal to 6 hours. At a high temperature of 900-950℃, the austenite grains on the surface of the steel pipe will continue to grow. When the calcination time exceeds 6 hours, the grain size will increase exponentially. The excessively long high temperature causes carbides to aggregate along the grain boundaries, forming network carbides such as Cr. 23 Substances such as C6 reduce the impact toughness of steel pipes by 40%. Therefore, in this application, after Rx gas is introduced again, the temperature of the steel pipe calcination furnace is lowered to 870℃-890℃ for a duration not exceeding 1 hour, so that the total processing time is less than or equal to 6 hours, in order to improve the stability of carbon on the surface of the steel pipe. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0021] Example 1: A method for preventing decarburization of steel pipes involves first connecting an Rx gas generator to a steel pipe calcining furnace via a pipeline. The pressure inside the calcining furnace is maintained at 35 Pa, and the oxygen content is ≤3 vol%. The furnace has a volume of 45 m³. In this embodiment, the furnace holds 180 steel pipes stacked in three layers. The spacing between adjacent pipes in the same layer is 2 cm, and a 2 cm gap is maintained between the pipes to ensure sufficient diffusion of Rx gas and N2 between them, preventing carburization dead zones and maintaining carbon potential stability. Furthermore, the oxygen content inside the furnace must be ≤3 vol%. Insufficient spacing can lead to airflow blockage and exacerbate the risk of decarburization; therefore, a certain spacing is necessary to prevent carburization dead zones from appearing on the steel pipe surface.

[0022] Next, the first RX gas was prepared by introducing natural gas into the Rx gas generator to obtain Rx gas. The components of the Rx gas, by volume ratio, are Co: 20.5%, H2: 39.8%, Co2: 0.3%, and the remainder is N2.

[0023] Carbon monoxide (CO) reacts with the surface of steel pipe at 900–950℃, depositing carbon atoms on the surface of the steel pipe to compensate for carbon loss at high temperature and inhibit the formation of decarburized layer.

[0024] CO can reduce metal oxides, and CO can reduce oxides such as FeO to elemental iron, maintaining the surface activity of steel and increasing its hardness.

[0025] CO can be used to regulate carbon potential. The CO concentration directly affects the carbon potential inside the furnace. Appropriate CO can achieve dynamic balance of carbon potential and avoid overcarburization or decarburization.

[0026] The auxiliary function of hydrogen (H2) is that H2 can improve the reducing power. H2 is a strong reducing agent, especially at high temperatures, which can accelerate the removal of the oxide film on the surface of steel. It can also work synergistically with CO to improve the carburizing efficiency and ensure uniform carbon concentration on the surface.

[0027] Furthermore, H2 can inhibit the formation of carbon black. An appropriate amount of H2 can inhibit the deposition of carbon black (free carbon) produced by CO decomposition, thereby reducing furnace pollution and equipment wear.

[0028] Carbon dioxide (CO2) has a double-edged sword effect. As a source of decarburization risk, CO2 reacts with the surface of steel at high temperatures, consuming the carbon elements in the steel and exacerbating the thickening of the decarburized layer (the decarburized layer in traditional processes is ≥30 μm). Therefore, the CO2 content needs to be controlled within an extremely low range.

[0029] Controlling the amount of CO2 can regulate the atmosphere balance. A small amount of CO2 can participate in the reverse reaction to maintain the dynamic balance between carburizing and decarburizing.

[0030] The introduction of nitrogen (N2) can reduce the reactivity and decrease the carbon potential. The volume of the steel pipe calcining furnace is 45m³. The volume occupied by the three layers of stacked steel pipes is about one-third of the volume of the calcining furnace. Therefore, the effective space that RX gas can obtain in the steel pipe calcining furnace is about 31.5m³. In order to maintain the stability of carbon potential in the furnace, the following must be satisfied: Rx-min=α×V. Where: Rx-min is the required volume of RX gas, α is the displacement factor, and V is the volume of the effective space in the steel tube calcining furnace.

[0031] To ensure the carburizing reaction proceeds fully, α is set to 4.3 times. 4.3 × 31.5 m³ = 135.45 m³, rounded up to 136 m³.

[0032] Therefore, the Rx gas generator needs to produce 136 m³ of RX gas initially, and then continuously feed the 136 m³ of RX gas into the steel tube calcining furnace for the carburizing reaction.

[0033] The carburizing reaction of steel pipes requires a sufficient supply of RX gas. If the RX gas is insufficient, it may lead to a carburizing dead zone, causing the carbon potential to become out of control, resulting in an increased decarburized layer and substandard products.

[0034] For the first time, RX gas was introduced into the steel pipe calcining furnace and reacted with the steel pipe at 900℃. The carburizing reaction time of the first introduced RX gas with the steel pipe was 4.5h. Then, natural gas is introduced into the Rx gas generator again to produce RX gas. The composition and proportion of the Rx gas produced again are the same as those of the Rx gas produced in the first batch.

[0035] A small amount of RX gas is introduced into the steel pipe calcining furnace again to replenish carbon. In this embodiment, 15 m³ of RX gas is introduced, along with N2. The volume of N2 introduced into the steel pipe calcining furnace is twice the volume of the RX gas introduced again, that is, the amount of N2 introduced is 30 m³. The introduction of N2 can smooth the carbon concentration gradient on the surface of the steel pipe. The introduction of N2 can reduce the reactivity and carbon potential, avoid the surface carbon concentration from exceeding the limit during the secondary carburizing stage, and force carbon atoms to diffuse from the surface layer of the steel pipe to the core, preventing brittle carbides from precipitating in a network. After the introduction of N2, the temperature of the calcining furnace is controlled at 890°C and the furnace is shut down after 0.5 hours.

[0036] Example 2: For the first time, RX gas was prepared and natural gas was introduced into the Rx gas generator. The volume of the steel pipe calcining furnace was 45 m³. The volume occupied by the three layers of stacked steel pipes was about one-third of the volume of the calcining furnace. Therefore, the effective space that RX gas could obtain in the steel pipe calcining furnace was about 31.5 m³. In order to maintain the stability of carbon potential in the furnace, the following condition must be met: Rx-min=α×V. Where: Rx-min is the required volume of RX gas, α is the displacement factor, and V is the volume of the effective space in the steel tube calcining furnace.

[0037] To ensure the carburizing reaction proceeds fully, α is set to 4.5 times. 4.5 × 31.5 m³ = 142 m³, rounded to 142 m³.

[0038] Therefore, the Rx gas generator needs to produce 142 m³ of RX gas for the first time. The components of the Rx gas are calculated by volume ratio as follows: Co: 20.5%, H2: 39.8%, Co2: 0.3%, and the remainder is N2.

[0039] 142 m³ of RX gas was continuously introduced into the steel tube calcining furnace to carry out the carburizing reaction.

[0040] For the first time, RX gas was introduced into the steel pipe calcining furnace and reacted with the steel pipe at 945℃. The carburizing reaction time of the first introduced RX gas with the steel pipe was 4.9h. Next, natural gas is introduced into the Rx gas generator again to produce RX gas. In this embodiment, the composition and proportion of the Rx gas produced again are the same as those of the Rx gas produced for the first time in this embodiment.

[0041] A small amount of RX gas is introduced into the steel pipe calcining furnace again to replenish carbon. In this embodiment, 15 m³ of RX gas is introduced, and N2 is introduced at the same time. The volume of N2 introduced into the steel pipe calcining furnace is twice the volume of RX gas introduced again, that is, the amount of N2 introduced is 30 m³. After N2 is introduced, the temperature of the calcining furnace is controlled at 870°C and the furnace is shut down after 0.8 hours.

[0042] Traditional processing techniques take more than 6 hours to complete, and the amount of Co2 is not strictly controlled, resulting in steel pipes with low hardness, high decarburization layer thickness, and poor quality.

[0043] Table 1: Comparison of Surface Structure Parameters of Steel Pipes Processed by Different Techniques As shown in Table 1, the proportions of each component in the RX gas differ slightly between Example 1 and Example 2. Additionally, the temperature difference before and after re-introducing RX gas in Example 1 was 60°C, while in Example 2 it was 75°C, resulting in a 15°C temperature difference between the two examples. The total processing time of Example 2 was longer than that of Example 1, but still within the range of 6 hours. The carbon concentration and surface hardness of the steel pipe in Example 2 were slightly higher than those in Example 1, but the difference was not significant. This difference mainly stemmed from the different total RX gas introduced and reaction times. The total RX gas introduced in Example 1 was less than that in Example 2, and its total reaction time was also less than that of Example 2.

[0044] The total reaction time of both embodiments is less than 6 hours, which aims to prevent the growth of austenite grains on the surface of the steel pipe and to control the thickness of the decarburized layer. Compared with the traditional process, the parameters of Embodiment 1 and Embodiment 2 are superior to those of the traditional processing process.

[0045] In the processing of Example 1 or Example 2, the total processing time is less than 6 hours. At a high temperature of 900–950℃, the austenite grains on the steel pipe surface continue to grow. When the calcination time exceeds 6 hours, the grain size increases exponentially. Prolonged high temperature causes carbides to aggregate along grain boundaries, forming network carbides such as Cr. 23 Substances such as C6 reduce the impact toughness of steel pipes by 40%. Therefore, in Examples 1 and 2, after Rx gas is introduced again, the temperature of the steel pipe calcination furnace is reduced to 870℃-890℃ for a duration not exceeding 1 hour, so that the total processing time is less than or equal to 6 hours, in order to improve the stability of carbon on the surface of the steel pipe.

[0046] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A processing method for preventing decarburization of steel pipes, characterized in that, Includes the following steps: S1: Connect the Rx gas generator to the steel tube calcining furnace via a pipeline; S2: Natural gas is introduced into the Rx gas generator to produce RX gas for the first time; S3: RX gas is introduced into the steel pipe calcining furnace and reacts with the steel pipe at 900–950℃ to undergo a carburizing reaction; S4: Natural gas is introduced into the Rx gas generator again to produce RX gas, and RX gas is introduced into the steel pipe calcining furnace again, while N2 is introduced at the same time to smooth out the carbon concentration gradient on the steel pipe surface. S5: After introducing N2, control the temperature of the steel pipe calcining furnace at 870℃-890℃ and shut down the furnace after 0.5-1 hour.

2. The processing method for preventing decarburization of steel pipes as described in claim 1, characterized in that, In step S1, the pressure inside the steel pipe calcining furnace is maintained at 30–35 Pa, and the oxygen content is ≤3 vol.

3. The processing method for preventing decarburization of steel pipes as described in claim 1, characterized in that, In step S1, the cavity volume of the steel pipe calcining furnace is 45m³, which can stack and accommodate 180-200 steel pipes.

4. The processing method for preventing decarburization of steel pipes as described in claim 3, characterized in that, The steel pipes are stacked in three layers in the steel pipe calcining furnace, with the vertical spacing between adjacent steel pipes in the same layer being 1.8cm-2cm.

5. The processing method for preventing decarburization of steel pipes as described in claim 1, characterized in that, In step S2, the Rx gas generator produces 135m³-145m³ of Rx gas for the first time.

6. The processing method for preventing decarburization of steel pipes as described in claim 1, characterized in that, In step S2, the RX gas is an endothermic atmosphere, and its composition by volume ratio is Co: 19-21%, H2: 38-42%, Co2: 0.25-0.45%, with the remainder being N2.

7. The processing method for preventing decarburization of steel pipes as described in claim 1, characterized in that, In step S3, the carburizing reaction time of RX gas with the steel pipe is 4.5-5 hours.

8. The processing method for preventing decarburization of steel pipes as described in claim 1, characterized in that, In step S4, the Rx gas generated again in the Rx gas generator is 15 m³.

9. The processing method for preventing decarburization of steel pipes as described in claim 8, characterized in that, In step S4, the volume of N2 introduced into the steel pipe calcining furnace is twice the volume of Rx gas introduced again, and the N2 introduction amount is 30m³.

10. The processing method for preventing decarburization of steel pipes as described in claim 1, characterized in that, The total calcination time of the steel pipe in the steel pipe calcination furnace is less than or equal to 6 hours.

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