大型鍛件生產過程的質量控制【論文】
發布時間：2018/09/22
大型鍛件生產過程的質量控制
Quality Control inManufacturing Process of
Large Forgings
摘 要：大型鍛件生產過程的質量控制，一方面主要介紹大型鍛件常見的質量缺陷偏析、夾雜物與有害元素、縮孔與疏松、氣泡、鍛造裂紋、過熱、過燒與溫度不均、白點、組織性能不均勻及其對策，另一方面主要介紹各個生產環節的質量控制點和注意事項。針對大鍛件的質量問題，所產生的原因，各個環節嚴格控制各種參數，減弱大型鍛件固有的缺陷，確保鍛件的產品質量。
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.
關鍵詞：大型鍛件；質量；生產過程控制
Keywords: large forgings, quality, manufacturing
process and control
單位：邯鄲市紫山特鋼集團有限公司
Company: Handan
Zishan Special Steel Group Co. Ltd.
Quality Control in
Manufacturing
Process of Large Forgings
（因篇幅問題，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.
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
When the copper is heated in a
peroxidative atmosphere at a temperature of 1100 to
（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 (>
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.
1.6 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
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
3.3 Use special thermal hot charging
car and the temperature of hot charging is 800
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
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
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
5.6 Determine the time of
hydrogen diffusion.
5.7 The temperature of slow cooling in the
pit is not greater than
Conclusion: The
large forging is like a newborn baby and needed hundredfold care in the whole
forging process !
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