JPS62263950A - Ferrite ductile cast iron for high temperature use - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD This invention relates generally to ferritic iron compositions with low temperature properties, and more particularly to ferritic ductile iron compositions that are substantially insensitive to high temperature embrittlement.

Prior Developments Ferritic ductile iron is an important industrial alloy that has several advantages over steel products, including lower material costs and better castability. However, for high temperature applications, there is a tendency to choose steel products with superior mechanical properties. High temperature embrittlement (
ctevatedteIIlperaturerebrit
The phenomenon of thinness (ETB) is one of the reasons why ductile cast iron cannot compete with steel in many applications. It is particularly unsuitable for applications where good thermal fatigue resistance is required, such as in turbine casing applications, high pressure vessels, and engine parts.

Although it has been known for a long time that ferritic ductile cast iron becomes brittle at high temperatures, little research has been conducted to elucidate this phenomenon. It has been found that ETB manifests itself in a fairly narrow high-temperature region with a large impact on various physical properties. Such effects include a decrease in tensile strength, a decrease in tensile ductility, and a decrease in low cycle fatigue resistance.

The cause of these negative effects on material properties is intergranular cracking Nnt
It has been confirmed that it is derived directly from the ergranular fracture.

It has also been observed that the temperature at which ductility is minimal is variable and dependent on strain rate. For the iron in question here,
The above temperature is 4 at a strain rate of 2.8 x 10-4 sec-'.
00°C (725'F), strain rate 1.4 x 10-
The temperature becomes 500°C (932'F) in 2 sec-'. However, even though this is known, until now the foundry industry has
No solution to the TB problem has been found.

Therefore, there is a need for a ferritic ductile cast iron that exhibits excellent high temperature properties including excellent tensile ductility, excellent low cycle thermal fatigue performance, and suppressed intergranular cracking.

SUMMARY OF THE INVENTION The present invention was developed to overcome the temperature-related shortcomings of conventional irons and provides a novel ferritic ductile cast iron with excellent strength, ductility and thermal fatigue resistance at high temperatures. After extensive testing of the effects of chemical composition, strain rate, and temperature, we have discovered that it is possible to produce a special ductile iron composition that is virtually insensitive to ETB and exhibits excellent fatigue properties comparable to conventional steel.

Comparing the characteristics of ETB with metal-induced embrittlement, both phenomena have an onset and recovery, and are influenced by strain rate.
I found that they are similar. It has been found that the decrease in mechanical properties associated with ETB is a direct result of intergranular cracking occurring when maximum loads are reached.

The mechanism for producing ETB appears to be magnesium-assisted segregation of sulfur. This mechanism is confirmed based on the correlation between magnesium, sulfur, and ETB generation. As a result of this knowledge, in accordance with the present invention, ferritic ductile iron compositions with low sulfur and/or magnesium levels are formulated to be substantially ETB insensitive.

The occurrence of ETB is determined by the ductility ratio (ductllltV ra
tto )". "Ductility ratio" is the value obtained by dividing the high temperature ductility measured at 425° C. (800 teeth) by the ductility at room temperature.

The ductility ratio distinguishes the effect of ETB on ductility from other effects. Alloying and microstructure affect both room temperature and high temperature ductility, leaving the ductility ratio unchanged. However, E
TB only affects hot ductility and therefore changes the ductility ratio significantly. If we rely only on high temperature ductility as an indicator of ETB,
Interrelationships become very weak as they are blurred or obscured by alloying and microstructural effects.

Most of the ferritic ductile irons currently available on the market are susceptible to ETB, which limits their temperature performance. By excluding ETB as taught by the present invention, superior alloys are obtained with high temperature fatigue resistance comparable to plain carbon steels and low alloy steels.

Accordingly, it is an object of the present invention to provide ferritic ductile iron compositions that are substantially ETB-insensitive by controlling and limiting the residual concentrations of magnesium and sulfur.

Various objects, features and advantages of the present invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS High temperature embrittlement (ETB) in ferritic ductile iron
) To better understand the phenomenon, detailed studies were conducted to investigate the effects of chemical composition, strain rate and temperature on ETB. Twenty-six different material compositions representing a wide range of ferritic ductile iron chemistries were considered. Each heat! The chemical composition of * is shown in Table 1, and the heat treatment and microstructure of each heat is shown in Table 2.

The general mechanical behavior of ferritic ductile iron in ETB generation pattern was investigated by tensile and low cycle fatigue tests. Tensile tests were conducted on four series.

The first series used standard ASTM methods to
m specimens were tested in air from room temperature to 650°C (1200'F).

Particular attention was paid to heats B, E, G, and H. The strain rate in these tests was 8.3 x 10'5ee to yield.
-', and it took 8.3×10-“sec” to break.

Microscopic examination of fractured specimens from tensile and fatigue tests was performed using longitudinal cutting, scanning electron microscopy, relief replica transmission electron microscopy and energy dispersive X-ray analysis.

Figure 1 shows heats B1E and G from the first series of tensile tests.
The area reduction for and H is shown as a function of temperature.

The ETB generation pattern is clearly visible at 425°C (below 800°C). It is important to note that heat H is substantially unaffected by ETB and exhibits clearly lower brittleness than the other heats.

The fracture stress data plotted in Figure 2 does not make it clear whether it is an ETB generation pattern. Still, even though the fracture stress drops sharply with increasing temperature in all heats, the fracture stress in Heat H, which is the preferred heat, is higher than the others. When the specimens tested at 425°C (below 800°C) for Heat 1-G and H were metallographically cut out, as shown in Figure 3, it was shown that there were intergranular cracks in Heat G. As shown in Figure 4, nothing was found in H1-H.

Room temperature and 425'C (8
From the tensile test, the ratio of ETB minimum area reduction to room temperature area reduction, i.e., the ductility ratio rD RJ (ductili
ty ratIo ) is the magnesium (Mg
) and the total sulfur (S) content. This correlation is shown in FIG. Depending on the DR value, ETB
The effect is clearly distinguishable from changes in mechanical properties due to alloying or microstructure. At a ductility ratio near 1, behavior similar to Heat H appears, and at a ductility ratio close to 1/4, behavior similar to Heat G appears. Low ductility ratio is high - and S
It is clear that there is a connection with content. By keeping the parameter (Mg+4.58) below 0.07,
ETB can be avoided. This represents a triangular area as shown on the left side of FIG. ETB behavior remains - and S
Due to the fact that these two
It is strongly suggested that two elements play a leading role.

Also shown in FIG. 5 is data from a heat containing modified graphite (ASTM type ■). Such an organization is
Usually Mg [fade] reflects J or insufficient addition and is insensitive to ETB but cannot be adopted.

The second series of constant displacement tensile tests were carried out by Greeple (01eeb
le) It was carried out using a 1500 test machine. Test data provides evidence that strain rate affects ETB. Figure 6 shows area reduction data for heats G and H.

This figure also shows that Heat H has relatively good ductility and a low minimum temperature. Furthermore, it can be seen that the temperature of minimum area reduction is 130°C (235'F) higher at higher strain rates.

Moreover, ETB temperature range (range where area reduction is small)
is larger at higher strain rates. Figure 7 shows the effect of strain rate on fracture stress.

The difference in embrittlement seen between heat G and heat H (among other things) was determined by fractography as shown in Figure 8A.
, B is related to the degree of intergranular cracking. The significant embrittlement of Heat G is associated with the high number of intergranular cracks and the presence of shrinkage bores within the intergranular crack regions. Figures 9A and 9B show the classical dimpled fracture behavior prevailing in heat H, which is relatively less brittle.

The range of intergranular cracking is shown quantitatively in Table 3, and is plotted in FIG. 6 for a large number of test pieces using the Grieple tester. A typical transmission electron microscope (TEM) relief fracture surface metallographic photograph of a grain boundary surface tensile tested at 425°C (800'F) is shown in FIG. Similarly, the first
As shown in Figure 1, transmission electron microscopy fracture surface metallographic photographs confirm that the pores are a result of contraction during solidification.

In the third series of two-stage interrupted tensile tests, the intergranular cracking areas are clearly visible and it can be seen that such failure starts at the shrinkage pores. In this test, the test bar is first preloaded at 425° C. (below 800° C.) using the Shibekuwa AS TM tensile test method, and then the test is interrupted before the maximum load is reached. The preloaded bars are then tested at room temperature to develop fracture initiation behavior. Figure 12 shows a scanning electron microscope (SEM) fracture micrograph from one of these interrupted tensile tests. From these tests, it is clear that failure begins with condensation within the ETB warm-incubation range.

The results of the strain-controlled low cycle fatigue test are shown in FIG. A large difference in fatigue resistance is evident.

Heat with excellent low cycle fatigue resistance also has low high temperature embrittlement response. These results are based on the 1 and S correlations (
DR ratio).

Scanning electron microscopy fracture microstructure examination of heat B low cycle fatigue specimens reveals intergranular cracking and associated shrinkage pores, as shown in FIGS. 14A and B. In the heat H test, intragranular fracture (
Only the transgranular f'rature) is shown.

The above test results provide evidence regarding the ETB failure mechanism. The observation that intergranular cracking starts from contraction pores allows us to relate the fracture mechanism to the liquid fraction or segregation that finally solidifies. The 1 and S dependence of such destruction was also demonstrated. Since 1 is relatively insoluble in iron, segregation to grain boundaries should occur in the final solidified portion of liquid 12. A small amount of magnesium sulfide (MgS) should be present in the molten metal during solidification because 1 is usually combined and 1 is added to the molten metal to scavenge impurities such as sulfur. The amount of IS present at grain boundaries depends on many factors, including molten metal chemistry, magnesium addition, and time.

The time factor explains the benefits of magnesium “disappearance”;
As a result of magnesium loss, ASTM type ■graphite is produced,
Improves fatigue resistance. Despite these variables, residual amounts of magnesium and sulfur can be directly related to ETB generation or insensitivity.

Once formed, MgS is unstable at temperatures above room temperature and decomposes into magnesium oxide (rlkIO) and free sulfur over time upon cooling from solidification or during heat treatment. At mid-day, the boiling point of sulfur is 455°C (855”F);
As if by coincidence, the temperature is close to the temperature allowed for ETB. All of the above suggests that ETB is the result of magnesium-assisted segregation of sulfur, with sulfur acting as a embrittling agent and magnesium as a transport mechanism. This appears to explain the interrelationship or synergistic relationship between magnesium and sulfur, where lower sulfur levels allow higher levels of magnesium to develop ETB.

The effect of strain rate can be seen more clearly in Figure 16. FIG. 16 shows the relationship between temperature and strain rate of minimum area reduction for heat G. The onset and recovery temperatures can be identified as the temperatures at which heats G and H exhibit significant area reductions above and below the minimum area reduction. From FIG. 16, the effect of strain rate on ETB heat can be more clearly understood. This behavior is similar to metal-induced embrittlement.

The temperature at the onset of ETB can be related to the mobility of sulfur, which alone should be diffusion controlled. In other words, in order for intergranular cracks to make a large contribution to the overall fracture process, sulfur must have a high damage rate.
must have sufficient mobility to keep pace with the At low temperatures and/or high strain rates, sulfur is probably not sufficiently mobile to keep pace with the fracture rate;
Only ductile fracture occurs due to insufficient sulfur migration time.

Recovery occurs because free sulfur can be prevented from oxidizing and further embrittlement. This process is also diffusion controlled and should be dominant at the specimen surface.

That is, at high temperatures and long periods of time (slow strain rates),
The mobility of oxygen is sufficient to reach the intergranular crack front and prevent further crack propagation.

Whatever the mechanism of ETB, the M,,-8 correlation shows that low sulfur is extremely important. Lowering the sulfur level increases the range of acceptable ruber and reduces the tendency to produce modified graphite.

It should be understood that the present invention is based on the identification of heat in a large number of ferritic ductile cast irons that do not generate ETB. Of particular interest is Heat H, which is virtually insensitive to ETB. A comparison of some of the fatigue properties of these heats with two commonly used steels is shown in Figure 17.

The fatigue performance of ductile iron heat is comparable to these steels.

Based on the test results described above, the following knowledge was obtained regarding the high temperature brittleness of ferritic ductile cast iron.

1) High temperature brittleness acts to reduce tensile strength, tensile ductility and fatigue resistance.

2) The high temperature range in which ETB is observed depends on the strain rate. 4. At a strain rate of 4
4"F), but 2.OX 10-2sec-'
At strain rates of , the temperature range is 370°C (700'F) -
The temperature is 760°C (below 1400°C).

3) The temperature at which the tensile area decrease is minimum moves toward higher temperatures as the strain rate increases. 4.4×10-5sec
-' strain rate, the minimum area reduction is 400°C (
At a strain rate of 2.0 x 10'5ee-', the minimum occurs at 530'C (below 986).

4) Initiation and recovery occur similarly to metal-induced embrittlement.

5) Intergranular cracking is associated with a decrease in tensile ductility, and at the minimum ductility value, up to 35% of the fracture surface can be explained by intergranular cracking.

6) Intergranular cracking begins at shrinkage pores.

7) Ductility ratio (DR) - Area reduction at minimum ductility temperature (80
The ratio of area reduction (as RA) to room temperature is correlated with magnesium and sulfur levels. Relatively E at low magnesium levels and/or low sulfur levels
Becomes insensitive to TB. A DR value close to 1/4 is a perfect ETB
, and DR values close to 1 indicate relative ETB insensitivity.

8) ETB appears to be the result of magnesium-assisted sulfur segregation. To avoid ETB, reduce the residual levels (wt%) of sulfur and magnesium to (Mlll+4.
58) According to relationship (7), the total content must be maintained below 0.070% by weight.

9) Low cycle fatigue life, like tensile ductility, is greatly influenced by ETH.

Table 1 lists the chemical compositions of various ferritic ductile cast irons, with each component expressed in weight percent.

Table 2 shows the heat treatment and microstructure of ferritic ductile cast iron.

Table 3 shows the fracture characteristics of Hee 1-G and H.

0 3.54 2.8B O,880,570,
02,018,025P 3J8 2.70 1
．． 51 0.613 0.02. 014. 035Q
3.45 2.72 0.55 0.6B 0
．． 05. 018. 022.009
．． 031. 05B, 005. 025. 04
1. 020. 02B, 051-008.024
．． 040. 010.090R3,603,3
00,490,010,02,005,022S
3.47 3.00 0.50 0.64.0.02
．． 017. 028T 3.60 2.71
0.48 0.01 0.02. 008. 029U
Lf34 2.70 0.03 0.00 0
．． 2G, 030. 047V 3.5B
2. l1iOO,030,000,24,024,05
0w 3.6B 2.52 0.03 0.0
0 0.24. 022. 052X 3. [i
6 2.50 0.04 0.01 0.25. 02
8, O[ilY 3.84 2.1B
0.28. 020. 01[iZ
3.49 2.68 0.3g,
020. 018.007.027.010.
010.010.008.024.010. 0
10.010.009.034. OIO,010,12
0, O05,008,003,017,032,004
,004,002,O12,028,004,008,
007,018,027,003,01B,011.0
20. 03B, 00B, 008 P FA 99/
1Q SCA 5015
0RSCA 99/IS
SCA 99/IT
SCA 99/IU
SR99/I V SR99/I W SR99/I X SR99/I Y SCA 20/
80Z SCA 10
/90(1)SR: As-cast 595℃ stress relief annealing SCA: Subcritical annealing only (720℃) FA:
In view of the teachings set forth in the following, fully annealed, critical annealing at 900 DEG C., and subcritical annealing at 720 DEG C., it will be apparent that many modifications and variations of the present invention are possible. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

[Brief explanation of drawings]

FIG. 1 is a graph showing the tensile area reduction as a function of temperature for Heats B, E, G, and H; FIG. 2 is a graph showing heat-1-B,
Graph showing the fracture strength of E, G and H as a function of temperature. Figure 3 is a photomicrograph showing the metallographic fracture surface of a tensile specimen taken from heat G (4) at 425°C ( 800
Bottom) shows intergranular cracking and shrinkage pores tested in 200x magnification, nital = N! tal etch), Figure 4 shows Heater 1 tested at 425°C (800″F).
- Micrograph showing the metallographic fracture surface of a tensile specimen taken from H(4) (200x magnification, nital etch); A graph showing the dependence of
The ETB insensitive area is defined between the coordinate axes and the straight line 1+4.58=0.07, and FIG.
FIG. 7 is a graph showing the fracture stress as a function of strain rate and temperature for H-1-G and H; FIG. 8A is a graph showing the tensile area reduction as a function of strain rate and temperature for 700'F) and strain rate 4. 4 X
Micrograph of the metallographic fracture surface of the tensile test piece of Heat G (8) tested at 10-5 sec-' (magnification 200 (ratio)
Figure 8B shows the area of intergranular cracks and shrinkage pores at a strain rate of 4 at 3706C (700 below).
, 4 X 10-5 sec-'
Figure 9A is a micrograph (1000x magnification) of the metallographic fracture surface of the tensile test specimen of G(8) showing the area of intergranular cracks and shrinkage pores. 4.
He-1-H tested at 4 x 10'5ee-'
A micrograph (200x magnification) of the fracture surface of the metallographic structure of the tensile test piece (2), Figure 9B, is shown at 370°C (below 700°C) at a strain rate of 4.
Heat H tested with 4 X 10'5ee-' l:
A micrograph (1000x magnification) of the fracture surface of the metallographic structure of the tensile test piece (2) is shown in Figure 10. A transmission electron microscope (TEM) relief replica fracture of the intergranular crack area of the test piece of heat G (3) is Figure 11, a photo of the surface metallographic structure (10,000x magnification), shows the heat G (3
Transmission electron microscopy (TEM) relief replica fracture surface metallographic photograph (10,000x magnification) of the shrinkage zone of the test piece of 425°C (below 800°C). A micrograph (500x magnification) of the intergranular cracking area of test piece (9) showing the intergranular cracking and shrinkage pore areas. The graph showing the cycle fatigue behavior, Figure 14A, is a micrograph (75x magnification) of the metallographic fracture surface of the He-1-H specimen (1) subjected to a low cycle fatigue test at 425°C (800"F). , FIG. 14B is 425
Hea 1- low cycle fatigue tested at 800°F
Micrograph of the metallographic fracture surface of test piece B (1) (enlarged 5
00x), Figure 15A is a micrograph (75x magnification) of the metallographic fracture surface of the Hea 1-H test piece (1) subjected to a low cycle fatigue test at 425°C (800 pieces), Figure 15B is 4
Heat 1 low cycle fatigue tested at 25°C (800″F)
A micrograph of the metallographic fracture surface of specimen (1) of -H (enlarged 50 (1), Fig. 16 is a graph showing the influence of strain rate on the high temperature ductility of ferritic ductile cast iron, and Fig. 17 is a graph showing the effect of strain rate on the high temperature ductility of ferritic ductile cast iron. Heat H, I at °C (800 tons),
J and I (low cycle fatigue properties of cast steel A216WC
It is a graph comparing C and steel plate A316 G+・55 (lateral orientation). −25= FIG, 1 λ Biao Huang (zl burn) p FIG, 5 J / L '( FIG, 6 Scarcity °C FIG, 7 F! (B) 0.8 (B) FIG, 9 FIG , l○ 之, - 1; FIG, 11 FIG, 12 100 steps! 7 Rasaiku 1 Shigai FIG, 13 (A) (B) CB) FIG, I5 1G, I4 Elegance and °C IGI6

Claims (1)

[Claims] 1. A ferritic ductile cast iron composition with excellent high-temperature embrittlement resistance and having a residual sulfur content of 0.015% by weight or less. 2. The iron composition according to claim 1, wherein the residual amount of magnesium is 0.070% by weight or less. 3. The residual amount of magnesium and sulfur is (Mg+4.5S)
The iron composition according to claim 1, wherein the total content is 0.070% by weight or less. 4. The iron composition according to claim 1, wherein the iron composition is subcritically annealed or completely annealed iron. An iron composition according to claim 1 having a microstructure containing pearlite in an amount of 5.3-10% by weight. 6. The iron composition according to claim 1, which contains graphite. 7. ASTM type with graphite content of about 10-99% by volume
7. The iron composition of claim 6 containing Type I or Type II graphite. 8. The iron composition of claim 6, wherein said graphite contains about 1-90% by weight ASTM Type IV graphite. 9. The iron composition of claim 4, wherein the fully annealed iron is critically annealed at a temperature of about 702°C and subcritically annealed at a temperature of about 595°C. 10. The iron composition of claim 5, wherein said pearlite is present in an amount of about 3-5% by weight. 11. The alloy of claim 1, wherein the alloy has shrinkage pores in the range of about 1.1-6.2%.