EP0241812A2 - Ferritisches Kugelgraphit-Gusseisen für den Einsatz bei erhöhter Temperatur - Google Patents

Ferritisches Kugelgraphit-Gusseisen für den Einsatz bei erhöhter Temperatur Download PDF

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Publication number
EP0241812A2
EP0241812A2 EP87104872A EP87104872A EP0241812A2 EP 0241812 A2 EP0241812 A2 EP 0241812A2 EP 87104872 A EP87104872 A EP 87104872A EP 87104872 A EP87104872 A EP 87104872A EP 0241812 A2 EP0241812 A2 EP 0241812A2
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EP
European Patent Office
Prior art keywords
etb
iron
fracture
sulfur
iron composition
Prior art date
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Application number
EP87104872A
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English (en)
French (fr)
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EP0241812A3 (en
EP0241812B1 (de
Inventor
Thomas Raymond Farrell
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General Electric Co
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General Electric Co
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Publication of EP0241812A3 publication Critical patent/EP0241812A3/en
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C37/00—Cast-iron alloys
    • C22C37/04—Cast-iron alloys containing spheroidal graphite

Definitions

  • This invention relates generally to a ferritic iron composition having improved elevated tempera­ture properties, and particularly relates to a ferritic ductile iron composition which is substan­tially immune to elevated temperature brittleness.
  • Ferritic ductile iron is an important engi­neering alloy having several advantages over steel products, including low material cost and castability. However, for elevated temperature applications, steel products tend to be chosen for their superior mechanical properties.
  • the phenome­non of elevated temperature brittleness (ETB) is in part responsible for the inability of ductile iron to compete with steels in such applications. This is particularly true in cases where good thermal fatigue resistance is required such as turbine casing applications, high pressure vessels and engine components.
  • the present invention has been developed to overcome the temperature related drawbacks of conventional iron by providing a new ferritic ductile iron having improved strength, ductility and thermal fatigue resistance at elevated tempera­tures. Extensive testing of the effects of chemi­cal composition, strain rate and temperature have led to the discovery that certain ductile iron compositions may be produced which are substantial­ly immune to ETB and which exhibit improved fatigue properties comparable to several common steels.
  • ETB The characteristics of ETB have been found to be similar to metal induced embrittlement in that both phenomena exhibit onset and recovery and a strain rate effect. Reduced mechanical properties associated with ETB have been found to be the direct result of the development of intergranular fracture which develops upon reaching maximum load.
  • the mechanism responsible for ETB appears to be magnesium assisted sulfur segregation.
  • the identification of this mechanism is based upon correlations between magnesium, sulfur and the occurrence of ETB.
  • ferritic ductile iron compositions containing low sulfur and/or low magnesium levels have been formulated in accordance with the in­vention so as to be substantially immune to ETB.
  • ETB The occurrence of ETB is studied through examination of a value defined as a "ductility ratio" which is the elevated temperature ductility measured at 425C (800F) divided by the ductility at room temperature.
  • the ductility ratio separates ETB effects from other effects on ductility. Alloying and microstructures effect both room temperature ductility as well as elevated tempera­ture ductility so as to leave the ductility ratio unchanged. However, ETB effects only elevat­ed temperature ductility and thus dramatically changes the ductility ratio. Reliance on elevated temperature ductility alone as an indicator of ETB would provide a much weaker correlation due to the clouding or obfuscating effects of alloying and microstructure.
  • the general mechanical behavior of ferritic ductile iron in the ETB regime was studied with the aid of tensile and low cycle fatigue testing.
  • the tensile tests were in four series, the first series involving air testing 6.35 mm diameter test speci­mens from room temperature to 650C (1200F) using standard ASTM practice. Primary emphasis was on heats B, E, G and H. The strain rate in these tests was 8.3 x 10 ⁇ 5 sec ⁇ 1, up to yielding and than 8.3 x 10 ⁇ 4 sec ⁇ 1 to fracture.
  • Microscopic examination of failed tensile and fatigue test specimens was performed using longitudinal sectioning, scanning electron microscopy, relief replica transmission electron microscopy and energy dispersive X-ray analysis.
  • Figure 1 shows the reduction in area from the first series of tensile tests as a function of temperature for heats B, E, G and H.
  • the ETB regime is sharply delineated at 425C (800F). It is important to note that heat H appears substantially unaffected by ETB and clearly shows less brittle­ness than the other heats.
  • Figure 5 includes data from heats containing degenerate graphite (ASTM Type IV). Such struc­tures usually reflect Mg "fade” or an insufficient Mg addition, and may be unacceptable in spite of their immunity to ETB.
  • a third series of two-stage interrupted tensile tests provides a clear delineation of intergranular fracture areas and reveals that such fracture initiates at shrinkage pores. These tests involved first pre-loading the test bar at 425C (800F) using standard ASTM tensile test procedures and then interrupting the test prior to reaching maximum load. The pre-loaded bars were then tested at room temperature to reveal fracture initiation behavior.
  • Figure 12 shows a scanning electron microscope fractograph from one of these interrupt­ed tensile tests. It is clear from these tests that fracture initiates at shrinkage pores in the ETB temperature range.
  • a scanning electron microscope fractographic examination of a heat B low cycle fatigue specimen identifies intergranular fracture and associated shrinkage porosity as shown in Figures 14 a and 14 b .
  • Examination of heat H shows only transgranular fracture in Figures 15 a and 15 b .
  • MgS is unstable at temperature above room temperature and, with time, decomposes into magnesium oxide (MgO) and free sulfur upon cooling from solidification or during heat treat­ment.
  • sulfur has a boiling point of 455C (855F), which is coincidentally close to the temperatures observed for ETB.
  • ETB is the result of magnesium assist­ed sulfur segregation with sulfur as the embrittler and magnesium as the transport mechanism. This appears to explain the interrelation or synergy between magnesium and sulfur in which lower sulfur levels allow for higher magnesium levels before observing ETB.
  • strain rate effect can be seen more clearly in Figure 16 which shows a plot of the temperature of minimum area reduction versus strain rate for heat G. Onset and recovery temperatures can be identified from the temperatures at which heats G and H show comparable area reduction above and below the minimum. From Figure 16 a clearer understanding of the strain rate effect on ETB affected heats is obtained. This behavior has similarities to metal induced embrittlement.
  • the temperature for onset of ETB can be related to the mobility of sulfur, and as such should be diffusion controlled. That is, that in order for intergranular fracture to contribute significantly to the overall fracture process, sulfur must be mobile enough to keep pace with the damage rate. At low temperatures and/or high strain rates, sulfur is presumably not mobile enough to keep pace with the damage rate and only ductile fracture occurs because of insufficient time for the movement of sulfur.
  • Recovery may occur from the possibility that free sulfur oxidizes and is prevented from causing further embrittlement. This process would also be diffusion controlled and would be predominant at the specimen surface. That is to say that at higher temperatures and longer times (slow strain rates), oxygen may have sufficient mobility to reach intergranular crack fronts and prevent further crack advance.
  • the Mg-S correlation indicates that low sulfur is highly important.
  • the reduction of sulfur levels leads to wider ranges of acceptable Mg levels and to a lower propensity for degenerated graphite.
  • the present in­vention is based on the identification of a number of heats of ferritic ductile iron which do not exhibit ETB. Of particular interest is heat H which is substantially immune to ETB. A comparison of the fatigue properties of some of these heats with two commonly used steels is shown in Figure 17. The ductile iron heats are comparable in fatigue capability to these steels.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
  • Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)
  • Compounds Of Iron (AREA)
  • Casting Support Devices, Ladles, And Melt Control Thereby (AREA)
  • Soft Magnetic Materials (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Investigating Strength Of Materials By Application Of Mechanical Stress (AREA)
EP87104872A 1986-04-07 1987-04-02 Ferritisches Kugelgraphit-Gusseisen für den Einsatz bei erhöhter Temperatur Expired - Lifetime EP0241812B1 (de)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US84861286A 1986-04-07 1986-04-07
US848612 1986-04-07

Publications (3)

Publication Number Publication Date
EP0241812A2 true EP0241812A2 (de) 1987-10-21
EP0241812A3 EP0241812A3 (en) 1990-08-22
EP0241812B1 EP0241812B1 (de) 1993-09-08

Family

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Family Applications (1)

Application Number Title Priority Date Filing Date
EP87104872A Expired - Lifetime EP0241812B1 (de) 1986-04-07 1987-04-02 Ferritisches Kugelgraphit-Gusseisen für den Einsatz bei erhöhter Temperatur

Country Status (4)

Country Link
EP (1) EP0241812B1 (de)
JP (1) JPS62263950A (de)
KR (1) KR870010207A (de)
DE (1) DE3787302T2 (de)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2265154A (en) * 1992-02-18 1993-09-22 Cofap Nodular cast iron and method for making it

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1758038A1 (de) * 1968-03-23 1970-12-10 Rheinische Stahlwerke Zaehes Gusseisen mit Kugelgraphit im Gusszustand
GB2134135A (en) * 1983-01-24 1984-08-08 Ford Motor Co High-strength ferritic ductile iron
EP0131092A2 (de) * 1983-06-13 1985-01-16 Klöckner-Humboldt-Deutz Aktiengesellschaft Verfahren zur Herstellung eines Gusseisens mit Vermiculargraphit
GB2147007A (en) * 1983-09-27 1985-05-01 Ishikawajima Harima Heavy Ind Spheroidal graphite ferrite cast iron

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1758038A1 (de) * 1968-03-23 1970-12-10 Rheinische Stahlwerke Zaehes Gusseisen mit Kugelgraphit im Gusszustand
GB2134135A (en) * 1983-01-24 1984-08-08 Ford Motor Co High-strength ferritic ductile iron
EP0131092A2 (de) * 1983-06-13 1985-01-16 Klöckner-Humboldt-Deutz Aktiengesellschaft Verfahren zur Herstellung eines Gusseisens mit Vermiculargraphit
GB2147007A (en) * 1983-09-27 1985-05-01 Ishikawajima Harima Heavy Ind Spheroidal graphite ferrite cast iron

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
GIESSEREI-PRAXIS *
GIESSEREI-PRAXIS, Nr. 3,4, 1985, Berlin, D; K. ROEHRIG: "Eigenschaften von unlegiertem und niedriglegiertem Gusseisen mit Kugelgraphit bei erhöhten Temperaturen" *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2265154A (en) * 1992-02-18 1993-09-22 Cofap Nodular cast iron and method for making it

Also Published As

Publication number Publication date
KR870010207A (ko) 1987-11-30
JPS62263950A (ja) 1987-11-16
DE3787302T2 (de) 1994-02-24
DE3787302D1 (de) 1993-10-14
EP0241812A3 (en) 1990-08-22
EP0241812B1 (de) 1993-09-08

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