The birth, application and development trend of modern ductile iron (2)

4. Research on basic theory and basic technology of ductile iron

In terms of spheroidization theory, Harbin Institute of Technology, Institute of Physics of Chinese Academy of Sciences, Huazhong Institute of Technology, Zhenjiang Agricultural Machinery College and other units use liquid quenching, thermal analysis, scanning electron microscopy, electronic probes and other advanced means to test, and systematically put forward their own views. : Spherical graphite is precipitated directly from the liquid phase; graphite distortion may occur in liquid state; the structural mechanism of graphite formation grows in a spiral dislocation manner; the cause of graphite distortion is that surface active atoms such as oxygen and sulfur block the helix The dislocation step hinders the spherical growth of graphite; the spheroidizing elements such as magnesium and rare earth function as a "cleaning agent" for deoxidation and desulfurization. These views enrich the original spheroidization theory and have important guiding significance for the actual production problems such as the quality control of the original molten iron, the rational selection of spheroidizing agents, the strengthening of gestation, the prevention of spheroidization and the decline.

In terms of graphite morphology, Tsinghua University used scanning electron microscopy and other methods to study the graphite morphology and internal structure of various cast irons by means of deep etching and hot corrosion. A set of SEM traces of cast iron graphite morphology was established. This paper puts forward the document "Classification and naming of cast iron graphite", which deepens the understanding of the morphology of cast iron graphite.

In terms of the basic performance of ductile iron, Zhengzhou Machinery Institute, Beijing Iron and Steel Institute, Beijing Electromechanical Institute and other unit systems have measured the dynamic and static mechanics, use and processing properties of various bases and different grades of rare earth magnesium ductile iron. Iron materials provide scientific data for the production of harsh parts (such as gears).

Third, the latest development and development direction of ductile iron production technology

Looking at the development of ductile iron production and its technology in the past one or two decades, there are eight aspects of progress and trends as follows.

1. The production and proportion of ductile iron is increasing year by year, which is the largest increase in castings (see Table 1 ).

Table 1 Statistics of casting production in several countries in 1981, 1990, 1994 and 1997

country	year	Total casting output (t)	Cast iron		Gray iron casting		Ductile iron		Malleable iron casting
country	year	Total casting output (t)	Yield (t)	percentage (%)	Yield (t)	percentage (%)	Yield (t)	percentage (%)	Yield (t)	percentage (%)
China	1981	5371000	4476000	83	3971000	74	242000	4.5	263000	4.9
	1990	5589000	6854000	77	5433300	61	922400	10	498000	5.6
	1994	1126000	9293000	77	7607330	65	1321230	11	364590	3.0
	1997	11080442	8794256	79	6873736	62	1564066	14	354454	2
United States	1981	13889000	11205151	80	8826266	64	1996986	14	381899	2.7
	1990	10210000	7727000	76	4608000	45	2871000	28	248400	2.4
	1994	13267000	9756000	73.5	5831000	44	3691000	27.8	243000	1.8
	1997	14333000	10488000	73	6153000	43	4128000	29	207000	1.4
Japan	1981	6321221	5297000	83	3034302	48	1681742	27	280122	4
	1990	6804601	4996166	73	3324739	48	1415301	twenty one	256126	3.7
	1994	6740690	5199022	77	3035179	45	2016488	29	147355	2.2
	1997	7082427	5391349	76	3094535	44	2159115	30.5	137699	1.9
Germany	1981	4134421	3402837	82	2509169	61	746478	18	147190	3.5
	1990	4249117	3369000	79	2295000	54	955000	twenty two	119000	2.8
	1994	3815174	3050080	79.9	1992329	52	979231	25.6	78520	2.0
	1997	4120048	3239000	78.6	2054000	50	1136000	27.6	49000	1.2

It can be seen from Table 1:

a. The proportion of ductile iron production in major industrial countries has reached about 30%; China is about twice as low as industrial developed countries, but the fastest increase, the output has increased by 7 times, and the proportion has increased by 3 times.

b. The proportion of gray iron castings in the same period has been decreasing year by year, and the output and proportion of malleable castings have dropped significantly.

2. Adopting advanced technology to stabilize and improve the quality of large-scale production of ductile iron

The automotive industry is the main user of ductile iron, reflecting the highest level of production and production technology of ductile iron. The production of a large amount of ductile iron represented by automobile castings is recommended to adopt the following three sets of advanced technologies:

a. High-quality low-sulfur and low-oxygen high-quality raw iron water is the basis for stable internal quality.

Advanced technology such as casting coke, hot-air water-cooled cupola, electric furnace melting or double superheating, shaking or pneumatic desulfurization, and direct reading spectrometer for rapid analysis of components.

b. Special spheroidizing agents, inoculants and reasonable spheroidization and inoculation processes (with flow or type) are necessary means to ensure good spheroidization and matrix stability.

c. High-rigidity casting and advanced molding sand are effective measures to improve the quality of castings inside and outside.

The high compaction mold and resin sand such as static pressure, high pressure, gas pressure and pressure, and extrusion can effectively reduce the shrinkage, improve the dimensional accuracy of the casting and reduce the roughness.

In addition, in order to adapt to the multi-variety variable batch production mode, it is also necessary to apply technologies such as quick change template, quick-regulating gate, venting needle and sand quantitative.

Comprehensively adopting the above-mentioned complete set of technology, the process and equipment of China's major automobile foundries have reached the international level, and the production scale is also close to the international level (such as the annual output of the second steam injection plant No. 70,000 tons), and the static performance can reach QT450-10 stably. QT400-15, QT500-7, QT600-3, the degassing effect of the stable Nanqi cast state performance reaches QT600-10, after the normal fire can reach the high performance level of QT880-5.

3. Centrifugal cast pipe production has increased dramatically, becoming the largest user of ductile iron

Due to its high strength, good plasticity and good corrosion resistance, centrifugal ductile iron pipes are widely used in water supply and gas pipelines. Centrifugal duct iron pipe production in industrialized countries has accounted for 95% to 98% of total pipe production, and 40% to 50% of total ductile iron production, making it the largest user of ductile iron. The medium and small caliber (<1000mm) cast pipe adopts the water-cooled metal type process (ie, the cold die method); the large diameter adopts the hot die method (spray paint and lining resin sand). Foreign countries have been able to produce cast pipes up to DN2000mm x 8000mm and DN2600mm x 4000mm.

In the past ten years, China has introduced and developed a batch of centrifugal ductile iron pipe production lines with a production capacity of 700,000 tons and an annual sales volume of about 300,000 tons, accounting for more than 20% of the total amount of ductile iron. Among them, Xinxing Casting Pipe Group Co., Ltd. has the largest output, and has been able to mass-produce large-diameter cast pipes of DN (1200-1600) mmÃ—5000mm by hot-die method.

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