JPS624855A - Stainless steel cast alloy and its production - Google Patents

Abstract

Cast metal parts such as turbocharger housings which are subject to operating temperatures up to about 2000 DEG F (1093 DEG C) and require good thermal cyclic durability (resistance to thermal cracking) and burst containment properties, are made from a low nickel duplex stainless steel with nitrogen addition by the use of grey and ductile iron casting technology.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a stainless steel casting alloy and a method for producing the same.

The stainless steel casting alloy according to the invention is useful as a material for, for example, turbine housings, turbocharger shirts, exhaust manifolds, combustion chambers, etc.
It has good corrosion resistance and room temperature and high temperature properties within the operating range of °F (approximately 1093 °C).

(Prior Art) In general, turbocharger shirts and housings for automobiles and aircrafts are capable of handling high operating temperatures of up to 2000°F (approximately 1093°C) and rotating turbine impellers at extremely high speeds without causing damage. It must be configured so that it can be stored in

For example, in the turbocharger of a truck diesel engine, the temperature is between 1300 and 1400'? (about 7
04-76000), so the housing metal temperature reaches 1200-1300°F (approximately 649-704°a-
) becomes. On the other hand, automotive turbochargers have operating temperatures of up to 1750-2000°F (approximately 954-2000°F).
1093°C) t, so the gas introduction part of the turbocharger, that is, the tongue part, has a temperature that is not much different from the turbine exhaust gas temperature.The turbocharger has an adiabatic structure that prevents rapid dissipation of heat. The metal temperature at the tongue of the charger housing is 1550-1950°IF (
843-1065°C). Therefore, unless relatively expensive stainless steel casting alloys are used, there is a risk of thermal decomposition in metal parts, such as the gas inlet where the exhaust gases initially come into contact with the turbocharger.

Conventionally, turbine housings and other materials have been manufactured using 9Ni Re5ist, a chromium-nickel-iron stainless steel alloy with approximately 30 chromium, 20% nickel, and the remainder essentially iron, developed by the International Nickel Company.
Commercially available ductile high nickel cast alloys such as K30 are used.

Commercially available llID alloys containing approximately 26-30% chromium and 4-7% nickel have also been proposed, but they have a duplex structure i), and the nickel content of HD alloys is relatively low. A sigma phase forms in the HD alloy, making it extremely brittle, and thermal decomposition has occurred when used at high temperatures, especially when subjected to thermal cycling. On the other hand, HD like this
The drawbacks of the nickel-based alloys can be overcome by using stainless steel cast alloys with high nickel content, such as HK-based stainless steel alloys, which contain approximately 18-22% nickel and are entirely austenitic. , the strongest HK stainless steel alloy in terms of creep strength is Sten N1, which is 2-
10%, C! is 0.25-0.45%, Mn is 0.0%
A stainless steel wrought alloy with 1-2.5% and 0.35-0.55% N has been proposed, in which case the commercially available 21
Reducing the carbon and manganese content of -4 stainless steel alloys provides high temperature strength, sulfidation resistance, and oxidation resistance.

(Problems to be Solved by the Invention) However, these alloys that can satisfy the high-temperature property conditions required for turbocharger housings are generally quite expensive, and because of their high nickel content, they cannot be manufactured by casting. The problem was that it became complicated.

In addition, all of the wrought alloys in the above-mentioned US patents had an austenite phase, and did not have particularly low thermal expansion characteristics.

It is therefore an object of the present invention to
The object of the present invention is to provide an inexpensive stainless steel casting alloy that has excellent thermal decomposition resistance and room temperature strength at operating temperatures up to 5° C., has high creep strength, is rich in fracture resistance, and is inexpensive.

Another object of the present invention is to provide a low cost stainless steel casting alloy that exhibits excellent casting properties.

Still another object of the present invention is to provide a method for inexpensively and effectively casting stainless steel products that can be used at high temperatures.

(Another Means to Solve the Problems) According to the present invention, the above object is achieved by using a stainless steel casting alloy to
It has two metallographic phases of substantially 20-80% ferrite and the remainder austenite, 1500-1950
Excellent thermal decomposition resistance when subjected to heat cycling between the service temperature of 815 °F (approx. an elongation of at least substantially 7% at 75,000 psi and substantially no sigma phase;
Substantially 27-31% by weight of chromium and 4-6% of nickel
% by weight, 0.2-0.5% by weight of nitrogen, 0.2-0.2% by weight of carbon
0.4 i! %, Kornbuum is 0.5-1.511
: t%, up to 1.0% by weight of sulfurized products selected from the group consisting of manganese and molybdenum, 0.2 to 0.0% sulfur.
This is achieved by containing 4% by weight and the balance being iron.

(Function) According to the present invention, a dubrex stainless steel alloy,
That is, it is possible to obtain a two-phase alloy with both ferritic and austenitic structures, which can be used as cast parts subjected to high temperatures such as automatic turbo charger housings, gasoline engine exhaust manifolds, casting furnaces and combustion chambers. Thus, it is possible to obtain an effect that combines the high-temperature characteristics of the austenite phase and the low thermal expansion characteristics of the ferrite phase.

(Example) In the present invention, the stainless steel casting alloy suitably used for stainless steel casting products, especially turbine housings, is formed of H-series stainless steel material with a relatively low nickel content, and nickel combines with nitrogen to create ferrite. phase is 20~
80%, preferably 40 to 60%, and the remainder is modified into a ferrite-austenite two-phase structure with austenite phase, improving heat resistance. The amount of ferrite present within the microstructure of the alloy is determined by the alloy chemistry, fabrication techniques, and heat treatment methods employed. It is believed that the ferrite phase does not contribute to the high temperature properties of the cast alloy.

Without solution processing, the stainless steel cast products of the present invention are extremely brittle and require solution processing. At this time, the productivity of the steel casting method of the present invention can be increased by utilizing the fact that the cast steel is brittle and brittle before heat treatment. That is, the sprue of cast steel can be removed simply by breaking it off, without using a machine or processing. Solution processing is carried out at temperatures between 2000 and 2200°F (approximately 1
093-1204' (: 1-4 hours, followed by air cooling. After this solution treatment, the alloy remains at 1400-1600 degrees Fahrenheit for up to 24 hours.
It is desirable to perform the strengthening treatment at a temperature of 60 to 871 °C.
It will be appreciated that this strengthening treatment may be performed during test use of the alloy product at the beginning of its use since the alloy product will normally be subjected to temperatures in this range during use.

The stainless steel cast products of the present invention are strengthened primarily by carbides dispersed within a solid solution strengthened matrix. MO of the two types of carbides produced, namely MC and M23C6
Carbides (M substantially C,) I/i are relatively unaffected by solution processing and remain as reinforcing components after solution processing. On the other hand, the brittle M23Cfl component (M is essentially Or)
becomes spheroidal during solution treatment (apher-01d1ze)
Or partially dissolved. This dissolved carbide usually precipitates at low temperatures, improving the strength of the alloy. That is, by solution treatment, the MisOs carbide is redistributed, ie, spheroidized or dissolved, and the spheroidized or dropletized M2RCB carbide becomes more ductile than in its original angular form.

To enhance machinability, 0.2-0.4% sulfur is added to the stainless steel casting alloy of the present invention and combined with manganese or molybdenum to form MnB or MoB. Also, silicon, which acts to increase the fluidity of cast alloys, is typically present in up to two silicones in commercially available steels. , while 2.5 to 1
．． 5% Cornbium (Obium) is added to increase strength. Cornbium has an extremely stable MO
Produces carbide.

Another feature of the stainless steel alloy according to the invention lies in the casting process. That is, a low-friction casting method, such as that normally employed when casting ductile gray iron, is effectively employed.

Mainly steel casting is approximately 3100'lF (approximately 1700°C)3
Since iron casting is cast at a high temperature of about 2600-2900°F (about 1427-1593°C), steel casting is more expensive than iron casting.

In the present invention, stainless steel is used at approximately 2850°F (approximately 1
It has been found that casting can be performed at a tap temperature (the temperature at which stainless steel is transferred to the cauldron) of 566°C. Furthermore, when increasing the number of sprues to reduce porosity, the configuration normally employed in steel casting can be employed in casting the alloy of the present invention, and the quality of the cast product can be significantly improved. The alloy products cast according to the present invention have a unique chemical composition and microstructure, and the presence of brittle carbide components, which are partially dissolved during solution processing, can be removed by breaking off the sprue. In other words, since this carbide component is present in the continuous pouring state, the sprue can be removed by simply removing the sprue instead of using the costly machining process normally used in austenitic steel casting. can be removed.

Experimental Examples Turbine housings were cast using various DMS 016 alloys shown in Table 1 according to the present invention, and the characteristics of these turbine housings were tested. The results are shown in Table H. Table I also shows He, H, which is close to the DMSO16 alloy.
D and HE alloys are also listed.

The casting temperature is approximately 2733-27700F for 12 pieces.
(approximately 1500 to 1521°C). A commercially available mixture having close to the desired chemical properties of the DMSO16 alloy according to the present invention was used as the hot water material.

Figure 1 shows the microstructure of DM8016 alloy modified with 0.16% N and containing approximately 10% austenite.
Shown at 0x magnification. The bright parts in the figure are the austenite phase, and the dark parts are the ferrite phase. Also shown in FIGS. 2-4 are the microstructures of different alloys (0.20, 0.32 and 0.35 N, respectively) and about 20% austenite, respectively.

40-50% and 50-55%.

Table I O1,5-2,30,5(max),2-. 6Cjr
1.6-2,2 26-30 24
-28Ni 34-38 4
-7 18-22Mn 0.7
(Max) 1.5 (Max) 2.0 (Max) s14.
s -5,32,0 (max) 2.0 (max) MO
o, s (max) o, s (max) SO, 06 (max) 0.01 (max) 0.04 (max) cb
--- N, 02
, Q2 −Fe remainder
Remaining amount Remaining sipherite% −−
★ Austenite section --- ★ Basically, austenite axis with ferrite content as low as 10% Line intercept estimation method using micrographs (1ine
1ntercept θθt im.

*** Mg is 0.035 to 0.09, 1-. 2
Q, 19. 16
．． 2428-32 29.65
28.39 30.944, -8
5.47 5.21 5,
331.0 (maximum) 0.67. 49
．． 652.0 (maximum) 1.72 1.
49. 900.1 (maximum) 0.1 (maximum), 12. 34--. 1
4. 28--
−． 87-
0.2. 35
．． 32 remaining remaining remaining remaining -5
0★★ 40★★ 45★★ht
ion) The turbine housing is required to reliably house and hold the rotating impeller within the housing so that the turbo housing does not break. In the storage test,
It is examined whether the turbine housing can be reliably housed in a turbine housing made of a predetermined alloy under predetermined test conditions, even when the rotational speed of the impeller is increased and the impeller is disconnected.

- Bocharger manufacturers usually carry out this delivery test many times, for example on turbochargers for automobile (717) engines, diesel engines and aircraft engines. Tests on turbochargers for automobile and diesel engines are usually performed using the same relatively strong blades, but tests on turbochargers for aircraft engines
The test for the jar intentionally performs a containment test using a mechanically weak impeller. The alloy according to the invention, namely D
A storage test was conducted on MS0162. In accordance with the standard test guidelines for aircraft, where the shaft and impeller can easily break, one axially extending hole was drilled in the bar section and three holes were drilled in the back disk...the bulb section burst into three pieces. I set it up like this. The gas inlet temperature of the turbine was adjusted to 1750° F. (about 954° C.) at the turbine inlet 7 lange, and the turbine was rotated steadily at this temperature for 10 minutes at a speed of 97,500 rpm. Next, the turbocharger is rapidly accelerated until the weakened impeller ruptures to approximately 159 OOOr
It was broken at pm. It was found that even in this state, the destroyed impeller was reliably accommodated and retained within the housing. From this test result, the alloy DMSO1 according to the invention
It was found that the turbocharger housing made in 62 passed the same containment test for HK30+Ob, an austenitic alloy currently used in aircraft turbocharger housings.

Furthermore, a turbine housing αO for an aircraft of the known type shown in FIG.
A durability test of a 0-hour gas leaving cycle was conducted. As a result of inspecting the turbine housing after this test, no cracks were found in either the tongue portion (121) or the top portion α4 of the spiral portion (gas passage) surface.

It was found that this was cast with K alloy DMEIO162 and had excellent heat decomposition resistance.

An oxidation test was also conducted at 1500°F (approximately 816°C), but after 100 hours there was only a 0.03% weight loss.

A sulfidation test at 1700°F (about 927°C) was also conducted and only resulted in about 0.4% weight loss in one hour.

The alloy of the present invention can be used over a range of 300-1000"C.
s, e X 1 o-'/'c (10, I X 10
-'/°F). This coefficient of linear expansion is HK
30 stainless steel.

The element finite thermal stress model of the turbine housing shown in FIG. 5 was compared by casting the standard N1Reeist material (D-RI8) and the alloy according to the present invention. The results are shown in Table ■. As is clear from the irises on the surface, D
It was found that MSO162 had higher stress and longer fatigue life. In this case, the temperature of the tongue α2 is 152Q'?
(approximately 827°C), the temperature inside the sprue section μs to be discarded was 14,800F (approximately 804°C). Although these results are based on test data with limited creep (which refers to the strain that occurs as a result of applying a material test load and is dependent on the load-bearing time), according to the present invention, the creep test data As is clear from the creep test data shown in Table 11, the durability of the alloy DMSO162 according to the invention was found to be excellent.

Element finite stress analysis was performed on two critical surfaces where fatigue cracks are expected to occur, namely the tongue α2 and the sprue (to). With this, 9 DM80162 is D5S (
It was found that the strength and modulus of elasticity at high temperatures were higher than that of MiReieet) K, but the coefficient of thermal expansion was slightly lower. It will therefore be appreciated that the alloy according to the invention has high thermal stresses.

Five samples of DMSO16 containing different amounts of nitrogen (N) were prepared and mechanically tested. The test results are shown in Table ■. The elongation required to obtain satisfactory ductility is a minimum of about 7 inches, and it can be seen from the data in Table 1 that a minimum of 0.20% nitrogen (fN) is required. As is clear from Ebisu■, DMS with a nitrogen content of 0.20 or more
Cast samples of the 016 alloy were found to be substantially free of the brittle sigma phase. The maximum solubility of nitrogen (N) is about 0.6%, and when nitrogen (fN) is 0.5%, brittle nitrogen compounds appear and the ductility decreases.

House ! Ma C Ro Lo O~ ~ Pus
Watch~I -----he he cQ's
; ; ≦ ; ≦ ≦ ≦ ; ≦ ≦
For the turbine housings exposed to the above-mentioned tests and simulated or real environments, the DMSo 1 of the present invention
Compared to D5SNiResist, Alloy 6 has castability, machinability, and usability characteristics that are at least equivalent to or better than D5SNiResist, and are close to those of HK30 stainless steel or expensive Ni-nickel materials, and can be used under specified conditions. I think it's satisfying.

It will be understood that the present invention is not limited to the embodiments described above, but encompasses design modifications that fall within the technical spirit of the claims.

(Effects of the Invention) According to the present invention configured as described above, by adding an appropriate amount of nitrogen to low nickel duplex stainless steel, the thermal decomposition resistance can be improved and high nickel stainless steel can be obtained. The alloy can be effectively modified to a conventional configuration and provide substantially the same strength, corrosion resistance and creep properties as austenitic stainless steel. In addition, by adding nitrogen instead of nickel, the thermal decomposition resistance of the alloy, that is, the strength, can be increased, while the thermal expansion can be reduced, making it possible to make alloys that are functionally equivalent to stainless steel alloys other than HE. Realizes effects such as being able to provide low maintenance costs.

[Brief explanation of drawings]

Figures 1 to 4 are 0.16% and 0.20chi, respectively.
Microstructure of a sample from a turbine housing cast in DMSOI6 according to the invention containing 0.32 and 0.35% nitrogen (N) ft. FIG. 2 is a perspective view of a turbine housing model. 10... Turbine housing, 12... Tongue, 14
... Top, 16... Sprue part

Claims (10)

[Claims] (1) It has two metallographic phases of 20 to 80% ferrite and the rest is austenite, and 1500 to 1
It has excellent thermal decomposition resistance when subjected to heat cycling between the service temperature of 950°F (approximately 815-1065°C) and room temperature, has excellent oxidative corrosion resistance when subjected to solution treatment, and has excellent tensile strength at room temperature. elongation of at least substantially 7% at at least 75,000 psi, substantially no sigma phase, substantially 27-31% by weight chromium, 4-6% by weight nickel, and 0.5% by weight nitrogen. 2-0.5% by weight, 0 carbon
．． 2-0.4% by weight, Cornbium 0.5-1.5
% by weight, sulfurized products selected from the group consisting of manganese and molybdenum up to 1.0% by weight, sulfur 0.2-0.
Stainless steel casting alloy consisting of 4% by weight and the balance iron. (2) The stainless steel casting alloy according to claim 1, wherein the nitrogen content is 0.3 to 0.4% by weight. (3) The stainless steel casting alloy according to claim 1, which has a two-phase structure of 40 to 60% ferrite phase and the remainder austenite phase. (4) Substantially 31% by weight of chromium, 5% by weight of nickel, 0.24% by weight of carbon, and 0.65% by weight of manganese.
, 1% by weight of silicon, 0.35% by weight of molybdenum,
Sulfur is 0.3% by weight, Cornbium is 0.9% by weight,
2. A stainless steel casting alloy according to claim 1, comprising 0.32% by weight of nitrogen and the balance being iron. 5. The stainless steel cast alloy of claim 1 prepared by solution processing at substantially 2000-2200°F (approximately 1093-1204°C) for 1-4 hours. (6) After solution treatment, cooled with air for up to 24 hours, 1
6. A stainless steel casting alloy according to claim 5, which is prepared by a strengthening heat treatment at 400-1600°F (approximately 760-871°C). (7) Melt the steel mixture so that the chromium is substantially 27 to 3
1% by weight, nickel 4-6% by weight, carbon 0.2-0
．． 4% by weight, manganese 0.5-1.0% by weight, molybdenum up to 1.0% by weight, silicon 1-2% by weight, columbium 0.5-1.5% by weight, nitrogen 0.3%. ~
0.4% by weight, max. 0.03% phosphorus, 0.2% sulfur
~0.4 wt%, up to 0.50 wt% copper, and up to 0.20 wt% aluminum;
homogenizing the alloy by heating to a temperature of approximately 2850°F (approximately 1566°C);
) into a sprue-like mold that minimizes porosity at a tap temperature of approximately
A method for producing a cast stainless steel alloy having a two-phase structure of substantially 20 to 80% ferrite phase and the remainder austenite phase, comprising the steps of solution treatment at 4°C) and redispersion of M_2_3C_6 carbides. . (8) The manufacturing method according to claim 7, comprising the step of cooling the cast alloy to room temperature and then breaking off and removing the sprue before solution treatment. (9) The manufacturing method according to claim 7, wherein the cast alloy is cooled with air after solution treatment. (10) The cast alloy after solution treatment is substantially 1400-16
8. The method of claim 7, wherein the method is toughened at 00 DEG F. (760-871 DEG C.) for up to 24 hours.