大型锻件生产过程的质量控制

Quality Control inManufacturing Process of Large Forgings

 

摘 要：大型锻件生产过程的质量控制，一方面主要介绍大型锻件常见的质量缺陷偏析、夹杂物与有害元素、缩孔与疏松、气泡、锻造裂纹、过热、过烧与温度不均、白点、组织性能不均匀及其对策，另一方面主要介绍各个生产环节的质量控制点和注意事项。针对大锻件的质量问题，所产生的原因，各个环节严格控制各种参数，减弱大型锻件固有的缺陷，确保锻件的产品质量。

Abstract:Quality control of large forgings is a manufacturing process by which entities review the quality of all factors involved in production. On one hand, this paper briefly describes the research status of large forgings with the common flaws and their solutions, such as flaw segregation, inclusions & harmful elements, shrinkage cavity & porosity, bubble, discontinuities(crack), over-heating due to high-speed temperature raising in high-temperature sintering, uneven temperature, white spots, and uneven microstructure; On the other hand, it also describes the key quality control and the matters needing attention for each link of the manufacturing process. Aiming at the quality of large forgings, the reasons for creating all above quality problems of large forgings shall be carefully targeted, and various parameters shall be strictly controlled by each production link to reduce the flaws of the large forgings and to guarantee the quality of the products.

 

关键词：大型锻件；质量；生产过程控制

Keywords: large forgings, quality, manufacturing process and control

 

单位：邯郸市紫山特钢集团有限公司

Company: Handan Zishan Special Steel Group Co. Ltd.

Quality Control in

Manufacturing Process of Large Forgings

 

（因篇幅问题，中文原文略） 

I.  The common defects of large forgings

In the manufacturing process of large forgings, it is very easy to create some defects which are different from the modern and small forging. The reason can be various. For example, the size of forging section is large; the temperature always changes and the distribution of temperature is uneven when heating and cooling; the mobility of metal plastic is different; there are many defects in steel ingot. The defects included severe segregation and loose, dense inclusions, large columnar crystals and coarse uneven crystals, sensitive cracking and white points intendancy, grain heredity and temper brittleness, serious heterogeneity of structure properties.

The defects can be divided into several categories:

1、From the aspect of nature, the defects can be divided into five categories: unqualified chemical composition; unqualified structure property; decomposed second phase; porosity defects; cracks.

2、From the aspect of generation, the defects can be divided into two categories: the raw material defects during the process of smelting, tapping, injection, stripping cooling or heating; the forgings defects during the process of heating, forging, forging after cooling and heat treatment.

3、the common defects in large forgings are:

3.1 Segregation

Segregation refers to the uneven distribution of chemical composition and impurity in the steel. Generally speaking, it is called positive segregation when the chemical composition is higher than the average composition, whereas it is called negative segregation. There are macroscopic segregation，such as zone segregation, and microscopic segregation, such as dendritic segregation.

The segregation of the large forgings is closely related to the segregation of the steel ingot, and the segregation degree of the steel ingot is related to the type of steel and ingot, smelting quality and pouring conditions. The segregation is affected and aggravated by alloy elements, impurity content,and the gas in the steel. The greater the ingot , the higher the pouring temperature, the faster the pouring speed and the more severe segregation.

   （1）Zone segregation

The zone segregation is a type of macroscopic segregation, which is caused by the change of solubility and the difference of specific gravity when the molten steel chooses to crystallize during the process of consolidation. When the gas in steel floats up, the molten steel enriched with impurities will float up too, thus causing the strip trajectory which forms segregation like the whisker shape of ∧. When the top crystal and the refractory impurity sink, it will form segregation like the axis shape∨ as if the crystal rain falls. Precipitation at the bottom of the ingot forms negative segregation with sedimentary cone. The upper part of the solidification where is enriched with carbon, sulfur, phosphorus and other segregation elements becomes the normal segregation area with many defects.
The measures to prevent zone segregation are:

Reduce the content of segregation elements (such as sulfur and phosphorus)  and gas in steel. For example, using secondary refining, vacuum carbon deoxidation (VCD) treatment and argon blowing process at the bottom of ingot.

Use the measures of multi-furnace pouring, riser pouring, vibration pouring and heating and adiabatic riser to enhance the capacity of the riser filling.

Strictly control the injection temperature and injection speed, use the short and coarse ingot to improve the conditions of crystallization.

   （2）Dendritic segregation

    Dendritic segregation is a type of microscopic segregation. The heterogeneity between dendritic crystals and intergranular microdomain may cause uneven distribution of structure properties. Generally speaking, high temperature diffusion and heating, reasonable deformation in forging and homogenizing heat treatment can eliminate or reduce its adverse effects.

    3.2 Inclusions and harmful trace elements

    Inclusions can be divided into two kinds according to their sources: endogenous inclusions and foreign inclusions.
   3.2.1 The common endogenous inclusions mainly are sulfide, silicate, oxide and so on. Their number and composition in the steel are related to the composition of the steel, the quality of the smelting, the casting process and the deoxidation process. The endogenous inclusions with high melting point form the shape of regular angular since its solidification is prior to the base metal and the crystallization is not hindered; the endogenous inclusions with low melting point form the shape of ball, strip or dendrites since it is limited by solidified metal. When the ingot deforms after forging, the sulfide and silicate with better plasticity form the shape of strip when they extend in the direction of the main deformation. The endogenous inclusions with small size and scattered distribution are mostly less harmful due to their micro-defects. While inclusions with large or dense cloud-like distribution have the macro defects, which will create bad effects in the use of forgings and easily cause serious failure accident.

    3.2.2 Foreign inclusions are slag, protective slag, oxide film, refractory material and multi-metal blocks which are mixed into the steel. Often the foreign inclusions are thicker, and severe distribution will destroy the continuity of steel and cause scrap. As the development of large-scale equipment with high parameters, the quality requirements of large forgings become more stringent. Thus the lead, antimony, tin, bismuth, arsenic and other trace elements in the steel need to be controlled to improve the toughness of the forgings.
    The general measures to reduce the inclusions in steel are:
    1) Molten steel needs vacuum treating and secondary refining in order to control the quality of molten steel;

    2) Clean pouring to prevent foreign inclusions pollution and multi-metal come into the steel;
    3) Rationalize forging deformation and improve the distribution of inclusions.

    3.3 Shrinkage and rarefaction

    The porosity defect will damage the metal continuity, thus causing stress concentration and the source of crack, which are not allowed.
    At the time of ingot cogging, if the amount of resection is not enough and shrinkage and rarefaction are left, there will be tubular hole at the end of forging or serious center porosity. The shrinkage residue will appear when the shrinkage deepens into the spindle area and can not be completely removed in the process of forging due to low pouring temperature of the ingot and poorly compensated riser.

    The measures to prevent porosity defects are:
    1) Strictly control the pouring temperature and speed to prevent low temperature and slow injection;
    2) Use the exothermic riser or adiabatic riser and improve the feeding condition to make the shrinkage move to the riser area in order to prevent the shrinkage deepen into the ingot.

    3) Control the cut head rate of ingot riser and fully avoid the porosity defect. Rationalize the deformation and compact rarefaction defect.

    3.4 Blowholes

    Blowholes are divided into interior blowhole and subsurface blowhole:

    The furnace burden, the furnace gas and the air come into the steel. When the the deoxidation is badly treated in the process of smelting and the boiling exhaust is not sufficient, the gas in the molten steel will be too much. With decreasing temperature, the gas solubility will decrease and be segregated from the molten steel during the process of solidification, which causes the interior blowhole. When the die wall of ingot is wet and has rust, or there is moisture or volatile substances in the painting, the gas will be produced and penetrated into the surface of the ingot during the process of injecting high temperature molten steel, which forms the subsurface blowhole. Through  deformation in forging the blowhole will be flattened or expanded into cracks.

    The measures to prevent blowholes are:

    1）Fully bake the furnace burden and pouring system;

    2）Fully degas when smelting and use the protective pouring process;

    3）Weld porosity defect with high temperature diffusion.

    3.5  Forging cracks

    In large forgings, when the quality of raw materials or forging process is not good, it is often prone to forging cracks. Here are a few forging cracks due to poor material .

   （1）Forging cracks caused by ingot defects

    Most ingot defects will cause cracks when forging and the coefficient of line contraction is big. When the condensation feeding is insufficient and the difference between internal and external temperature is large, the axial with large tensile stress will crack along with the dendritic, which cause crack in crystal boundary. The crack will further develop into the spindle crack when forging. The defects can be eliminated with the following measures: 1.improve the smelting steel purity; 2.slowly cool the ingot and reduce thermal stress. 3. Use good heater and insulation cap to increase the ability of feeding. 4. Use the JTS forging process.

   （2）Forging cracks is caused when harmful impurities in the steel precipitate along the grain boundary.

    The sulfur in the steel often precipitates along the grain boundary in the form of FeS with a melting point of only 982 °C.  At forging temperature of 1200 °C, FeS at the grain boundary will melt and surround the grains in the form of liquid film, thus destroying the bond of grains and producing hot shortness, which will easily crack when slightly forging.
    When the copper is heated in a peroxidative atmosphere at a temperature of 1100 to 1200 °C, the surface layer will form a copper-rich region due to the selective oxidation. When the solubility of copper in austenite is exceeded, the copper is distributed in the form of liquid film along the grain boundary and  form copper brittleness , which can not be forged forming. If there is tin and antimony in the steel, it will also seriously reduce the solubility of copper in austenite and increase the tendency of this brittleness.

   （3）Forging cracks caused by outphase (the second phase) 

    The mechanical properties of the second phase in the steel are often different from those of the metal matrix, and thus it will cause additional stress and make the plasticity of the whole process decrease in the process of deformation flow. When the local stress exceeds the binding force of the outphase and the matrix, it will cause segregation and form the hole, such as  the oxides, nitrides, carbides, borides, sulfides, silicates and others in steel. If these phases are densely distributed and chain-like, especially distributed in the weak adhesion along grain boundary, it will crack at the high temperature of forging.

    The measures to prevent aluminum nitride segregate along the crystal and cause forging cracks are:

   1) Limit the amount of aluminum in the steel and remove the nitrogen in the steel or use titanium addition method to suppress the amount of AlN segregation.
   2) Use hot-rolled ingot and austenitic transformation process;
   3) Improve the heat transfer temperature (> 900 ℃) and directly heat the forging;
   4) Fully carry out homogenizing annealing before forging and make the precipitated phase of the grain boundary diffuse.

   3.6 Overheating, overburning and uneven temperature

    When the heating temperature is too high or the high temperature keeps too long, it easily causes overheating and overburning. Overheating significantly reduces the plasticity and impact toughness of the material. When overburning, the grain boundary of the material is violently oxidized or melted and completely looses the deformation capacity.
    When the heating temperature seriously distributes unevenly, the inside and outside, the positive side and the negative side of the forging blank along the length have very different temperature, which causes uneven deformation and eccentric forging. This phenomenon is also called under burnt. Figure 12 shows the bearing.

    The measures to prevent the heating defects are:

    1)Strictly carry out the correct specifications of heating;

    2) Pay attention to the charging way and prevent local heating;
    3) Adjust the temperature instrument, carefully carry out the heating operation, control furnace temperature and furnace gas flow and prevent uneven heating.

   3.7 White point

   The white point is an internal defect of forging which is caused in the cooling process after forging. In transverse macro coupon, it shows a sharp crack like hair and the fracture is silver white spot. The white spot is a kind of brittle sharp crack with great harm and is a kind of very dangerous defect in martensitic and pearlitic steel.

The white point is caused by the combined effect of hydrogen and additional stress. The hydrogen in the steel concentrates in the tension stress zone under the action of stress and make the steel produce so-called hydrogen brittleness and form brittle fracture.
   The measures to prevent the white points are:

   1) Reduce the hydrogen content in steel. The ways of reducing the hydrogen content in steel include: pay attention to the baking charge, fully boil when melting, vacuum degassing, secondary refining and so on.
   2) Use heat treatment to eliminate the white points. The main task is to diffuse hydrogen in steel and eliminate stress, such as using hydrogen annealing heat treatment.

    3.8 The nonuniform structure property

    Due to its large size, more procedures, long cycle and more uneven and unstable factors in the process, the large forgings often have serious nonuniform structure property, thus it can not pass the mechanical property test, microstructure inspection and non-destructive test. It will produce various porosity defects since the chemical composition in the ingot segregate and the inclusions concentrate; the slow temperature change, the uneven distribution and the large internal stress when heating cause more defects; due to high temperature of forging for a long time, plastic flow condition, compaction degree and deformation distribution are different; when cooling, the diffusion speed is slow, the structure transformation is complex and the additional stress is big. All of these factors may lead to serious nonuniform structure property and unqualified quality.

   The measures to improve the uniformity of large forgings are:

   1) Use advanced metallurgical technology to improve the metallurgical quality of steel ingots;
   2) Use controlled cooling technology and optimize the techniques process to improve technical and economic level in the production of large forgings.

   II. Quality control in manufacturing process of large forgings( As electric furnace smelting + LF refining + VD secondary refining + casting + forging as examples)

    1、The melting process

    1.1 All kinds of raw and auxiliary materials into the furnace must be fully baked.

    1.2 Use electric furnace smelting of concentrate (stainless quality carbon scrap) + pig iron or hot metal (25-30% of ingredients) ;the furnace door is equipped with coherent carbon-oxygen lance; strengthen smelting; timely make foam slag. 

    1.3 Control the aim carbon content and use as much as possible iron alloy or pig iron for carburization of the refining furnace.

    1.4 Prevent tapping slag and use the method of compound deoxidation which is conducive to the floating of deoxidation products and the removal of inclusions.

    1.5 In refining process, control the flow of argon, put an end to exposed molten steel and control the reducing atmosphere. Use micro-positive pressure and high alkalinity slag.

    1.6 Remove 2/3 of the reduction slag before vacuum pumping and keep vacuum degree below 67Pa for 15-20 minutes.

    1.7 Keep weak argon blowing for 10-15 minutes after VD and fully make  the inclusions float. Wire Feeding of Ca-Si alloy.

    2. Pouring system

    2.1 All raw and auxiliary materials which contact with molten steel must be baked and dried in pouring system.

    2.2 The temperature of ingot mold should be controlled at 30-80°C. Reasonably place the ingot mold.

    2.3 Use lightweight insulation caps and the protective slag should hang 1/2 and manually plus 1/2 when the molten steel is half up. Use argon protection pouring.

    2.4 Carefully control the temperature and rate of casting according to different types of steel.

    2.5 Use the cylindrical cooling-air system after pouring. The cooling-air strength should be in turn weakened from top to bottom: the head of steel ingot is largest, followed by the spindle and the tail is the weakest.

    2.6 The stripping should be in strict accordance with the solidification time of the ingot .

    3. Hot charging

    3.1 Large EAF forgings must be hot charging. On one hand, it can save energy, on the other hand it is more conducive to improving the quality of steel ingots.

    3.2 The temperature of material furnace must be controlled at 500-550 ℃.

    3.3 Use special thermal hot charging car and the temperature of hot charging is 800-850℃.

    4. The forging process of large forgings

    4.1 Use fully automatic high temperature fiber natural gas heater. Frequently observe the temperature and strictly control the heating speed especially at the temperature of 500-800 ℃.

    4.2 Strictly control the reduction amount and forging ratio.

    4.3 Forge strictly on the basis of the temperature of  initial forging and finish forging.

    4.4 Complete overalls tools.

    5. The heat treatment of large forgings

    5.1 Use fully automatic insulation fiber desktop furnace for annealing, with uniform temperature of electric furnace, better temperature control and better quality of heat treatment.

    5.2 Especially control the charging time and temperature after forging. If too early it is easy to cause coarse grain and too late to produce white points.

    5.3 Keep the forging into the furnace at the temperature of 350-400 ℃ for 3-5 hours to reduce the thermal stress.

    5.4 Strictly control the heating rate and uniform temperature.

    5.5 At the normalizing stage, use spray and air cooling. After normalizing, keep the forging at the temperature of 350-400 ℃ for 5-6 hours.

    5.6 Determine the time of hydrogen diffusion.

    5.7 The temperature of slow cooling in the pit is not greater than 50 ℃.

Conclusion: The large forging is like a newborn baby and needed hundredfold care in the whole forging process !

 

                                                2016-11-16