CA2320442A1 - Heat-resisting alloy with magnesium and calcium - Google Patents

Abstract

A heat-resisting weldable alloy consisting essentially of, by weight percent 0.1 to 0.4 aluminum, 0.001 to 0.008 calcium, 0.002 to 0.1 magnesium, 0 to 0.08 carbon, 0 to 1 manganese, 0 to 0.002 sulfur, 0 to 1 silicon, 0 to 0.75 copper, 0 to 0.3 phosphorus, 18 to 22 nickel, 18 to 22 chromium, and the balance iron and incidental impurities.

Description

HEAT-RESISTING ALLOY WITH
MAGNESIUM AND CALCIUM
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to improved process yield of a heat-resisting weldable wrought alloy, in particular an alloy of iron, nickel and chromium containing aluminum, magnesium and calcium.

2. Prior t Certain ferrous alloys including INCOLOY alloy 840 which include nickel and chromium, along with additives of aluminum and titanium, have been used to produce strip. These alloys are particularly useful for their heat-resisting properties.
An alloy such as INCOLOY alloy 840 strip should exhibit good hot malleability so that the steel may be hot rolled from 20 inch thick slab ingots to 0.250 inch gauge hot bands with minimal or no cracking. A typical composition of INCOLOY alloy 840 by weight percent is provided in Table 1:
Table 1 ALLOY 840 COMPOSITION (WT%) Carbon 0.08 max.

Manganese 1.00 max.

Sulfur 0.015 max.

Silicon 1.00 max.

Copper 0.75 max.

Nickel 18 to 22 Chromium 18 to 22 Aluminum 0.60 max.

Titanium 0.60 max.

Phosphorous 0.03 max.

Iron Balance Although INCOLOY alloy 840 has sufficient ductility to provide strip, a drawback to the use of titanium in the alloy is the formation of titanium nitride inclusions. These titanium nitride inclusions tend to cluster together and, during processing, form slivers and other surface defects. Prior attempts to produce a similar alloy without the addition of titanium have resulted in high process yield losses due to reduction in the alloy's hot malleability. The alloys of laboratory scale heats of Table 2 list the composition of various heats of modified alloys of INCOLOY alloy 840 which have been tested in an effort to reduce or eliminate titanium therefrom. Fig.
1 depicts a graph of hot ductility as measured by percent reduction in area for certain of these heats of modified alloys of INCOLOY alloy 840 having aluminum with or without titanium, calcium andlor cerium. Each of the heats produced without titanium has lower ductility than those which contain titanium. This is further illustrated in the Gleeble curve of Fig. 2 which compares the ductility (percent reduction in area) versus temperature of the laboratory heats shown in Table 2 annealed at 2200°F
tested on cooling from 2200°F.

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The results depicted in Figs. 1 and 2 demonstrate that the removal of titanium results in reduced hot ductility. Accordingly, a need remains for an alloy similar to INCOLOY alloy 840 with no or minimal titanium but with improved performance in a strip product.
SUMMARY OF THE INVENTION
This need is met by the alloy composition of the present invention which includes by weight percent 0.1 to 0.4 aluminum, 0.001 to 0.008 calcium, 0.002 to 0.1 magnesium, maximum of 0.08 carbon, maximum of 1.0 manganese, maximum of 0.002 sulfur, maximum of 1.0 silicon, maximum of 0.75 copper, 18 to 22 nickel, to 22 chromium, maximum of 0.03 phosphorous and the balance iron. It is believed that magnesium and calcium in controlled amounts sufficient to control the deoxidation process can be substituted for titanium added to INCOLOY alloy 840 and similar alloys to maintain the necessary hot ductility during rolling. The magnesium and calcium are conditioned and added during refining by first adding aluminum which is believed to bind with a portion of the available oxygen in solution. This improved deoxidation process has resulted in a product with improved internal cleanliness and improved surface appearance due to the absence of titanium nitrides in rolled out slivers.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a bar graph showing the ductility of various laboratory heats of modified alloys of INCOLOY alloy 840 listed in Table 2;
Fig. 2 is a graph of the percent reduction in area versus temperature of annealed laboratory heats of the modified alloys of INCOLOY alloy 840 listed in Table 2;
Fig. 3 is a graph of the percent reduction in area versus temperature of annealed production scale heats of modified alloys of INCOLOY alloy 840 and of alloys made in accordance with the present invention;
Fig. 4a is a photomicrograph (100 times magnified) of strip produced from an alloy made in accordance with the present invention;
Fig. 4b is a photomicrograph (500 times magnified) of the strip shown in Fig. 4a;
_4_ Fig. Sa is a photomicrograph (100 times magnified) of strip produced from conventional INCOLOY alloy 840; and Fig. Sb is a photomicrograph (500 times magnified) of the strip shown in Fig. Sa.
DETAILED DESCRIPTION OF THE INVENTION
The present invention includes a ferrous alloy containing nickel and chromium with minimal residual titanium to which is added aluminum, magnesium and calcium. Copending United States Patent Application Serial No. 091105,474, filed June 26, 1998, which is a continuation-in-part application of United States Patent Application Serial No. 08/943,293, filed October 14, 1997, entitled "Hot Working 'High-Chromium Alloy", discloses a nickel-chromium alloy including additives of magnesium and calcium which exhibits improved hot workability over prior high-chromium alloys for the production of nickel alloy wire. The present invention is based on the recognition that the same elements, magnesium along with calcium and aluminum, may be used as replacements for the titanium used in prior heat-resisting alloys to produce an alloy having acceptable hot ductility for the manufacture of heat-resisting strip. Moreover, the alloy of the present invention has been found to have improved weldability over INCOLOY alloy 840.
The preferred ranges for the amounts by weight percent of these critical additives in the alloy of the present invention are 0.1 to 0.4 aluminum, 0.001 to 0.008 calcium and 0.002 to 0.1 magnesium. Preferably, the alloy includes 0.15 to 0.25 aluminum, 0.002 to 0.006 calcium, 0.002 to 0.006 magnesium wherein the total of calcium and magnesium is 0.003 to 0.1. The preferred amounts by weight percent of the remaining elements of the alloy of the present invention are similar to that of INCOLOY alloy 840 or as follows: maximum of 0.08 carbon, maximum of 1.0 manganese, maximum of 0.002 sulfur, maximum of 1.0 silicon, maximum of 0.75 copper, 18 to 22 nickel, 18 to 22 chromium, maximum of 0.03 phosphorus and the balance iron.
The alloy of the present invention is made according to the following process. First, scrap metal containing at least the iron, nickel and chromium of the final composition is melted in an electric arc furnace in a conventional manner. This premelt is transferred to an argon oxygen decarburization {AOD) vessel where refining and alloying take place. At the deoxidation stage, aluminum is first added to the premelt in the AOD vessel to react with any residual oxygen in solution which would otherwise oxidize the magnesium and calcium to be added subsequently. After addition of the aluminum and any other alloying elements, the magnesium and calcium are added to the AOD vessel. The magnesium and calcium are believed to have a synergistic effect and act as effective substitutes for the titanium used in conventional INCOLOY alloy 840. The final molten composition is bottom poured into a 21 x x 90 inch slab mold or a 30 x 44 x 90 inch slab mold to form a slab ingot. The ingot is hot rolled to an intermediate size, surface overhauled and rolled in a hot reversing mill into a 0.250 inch thick coiled hot band. The coiled band is repeatedly pickled, annealed and cold rolled into 0.017 inch gauge strip. The resulting cold rolled strip may be used to produce heating element sheathing for electric stoves or other components requiring heat-resistant properties.
Although the invention has been described generally above, particular examples give additional illustration of the product and reference steps typical of the present invention.
EXAMPLES
Two production scale heats of alloys were made in accordance with the present invention as follows.
For each heat, scrap metal known to contain iron, nickel and chromium with minimal titanium was melted in a 35 ton electric arc furnace and transferred to an AOD vessel. Following the addition of conventional alloying elements to meet the specifications of INCOLOY alloy 840, aluminum bars were added to the AOD
vessel and melted. A calcium alloy addition containing about 5 % calcium and 95 ~
nickel as the metal portion thereof and a magnesium-nickel alloy were added to the AOD
vessel and melted. The resulting molten alloy was cast into a 21 x 43 x 90 inch slab ingot.
The ingot was hot rolled to an intermediate size, surface overhauled and rolled in a hot reversing mill into 0.250 inch thick coiled hot bands. The bands were repeatedly cold rolled, annealed at 1900-2100°F and pickled to a 0.017 inch gauge strip. The final composition of the alloy strips by weight percent were determined to be as shown in Table 3 , columns 1 and 2.
Table 3 Composit ion Weightrcent tion Heats Pe : Produc Element 1 2 M L N

C 0.022 0.032 0.028 0.021 0.021 Mn 0.340 0.370 0.350 0.360 0.39 Fe Bal Bai Bal Bal Bal S 0.0002 0.0009 0.0006 0.0007 0.0021 Si 0.60 0.64 0.64 0.60 0.65 Cu 0.18 0.32 0.21 0.35 0.24 Ni 18.30 21.84 21.11 19.81 21.14 Cr 18.25 18.48 19.58 19.59 19.59 A1 0.15 0.22 0.42 0.43 0.40 Ti 0.0(1 0.02 0.43 0.33 0.39 Mg 0.0030 0.0020 0.0000 0.0004 0.0011 Ca 0.0050 0.0030 0.0020 0.0020 0.004 COMPARATIVE EXAMPLES
Following the process outlined in the Examples, alloys containing titanium in accordance with conventional specifications for INCOLOY alloy 840 were prepared as a 0.017 inch gauge strip. The final compositions of the strips by weight percent was determined to be as shown in Table 3, columns M, L, and N.
Fig. 3 is a Gleeble curve of percent reduction in area versus temperature for alloys 1 and 2 of the Examples and alloys M and L of the Comparative Examples.
Figs. 4a and 4b are photomicrographs of the strip of alloy number 1 produced in the Example, polished, taken in the longitudinal direction of the strip at 100 times and 500 times magnification, respectively. Figs. 5a and 5b are photomicrographs of the strip of alloy N pr~luced in the Comparative Example, polished, taken in the longitudinal direction of the strip at 100 times and 500 times magnification, respectively.
The strip _7_ produced from alloy 1 has improved cleanliness over the strip produced from comparative alloy N. Surface observations of the strip produced from alloy 1 showed reduced slivers compared to the strip produced from comparative alloy N.
The alloy of the present invention also exhibits improved weldability over 1NCOLOY alloy 840 and similar alloys when used to fabricate seamed rolled rounds. Such products are particularly useful as outside sheathing material for electrical heating elements. The welding of seams of rolled alloys of the present invention may be performed (via tungsten - inert gas welding or plasma welding) up to thirty percent faster than welding of rolled alloys of INCOLOY 840. This higher throughput is due at least in part to improved cleanliness, improved arc stability and improved solidification behavior resulting in better weld profiles. In addition, tests on the bending of welded seams have shown that the new alloy is superior over the prior art alloys.
In particular, examination via scanning electron microscopy and metallography of welds of product formed from the alloy of the present invention have cleaner weld fusion zones and are free of titanium nitrides with reduced oxide formation over INCOLOY alloy 840. Differential thermal analysis revealed a higher solidification temperature for the new alloy as compared to the prior art alloys containing more titanium. This indicates that the alloy fusion zone solidifies at a higher temperature than the prior art alloys. The solidification temperature remains relatively narrow rendering the alloy resistant to solidification cracking. The weld beads formed have higher cracking resistance during subsequent forming operations resulting in fewer finished goods defects.
It will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed in the foregoing description. Such modifications are to be considered as included within the following claims unless the claims, by their language, expressly state otherwise.
Accordingly, the particular embodiments described in detail herein are illustrative only and are not limiting to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.
_g_

Claims (10)

We Claim:

1. A heat-resisting weldable alloy consisting essentially of by weight percent, about 0.1 to 0.4 aluminum, 0.001 to 0.008 calcium, 0.002 to 0.1 magnesium, 0 to 0.08 carbon, 0 to 1 manganese, 18 to 22 nickel, 18 to 22 chromium, and the balance iron and incidental impurities.

2. The alloy of claim 1 including 0.15 to 0.25 aluminum.

3. The alloy of claim 1 including 0.002 to 0.006 calcium.

4. The alloy of claim 1 including 0.002 to 0.006 magnesium.

5. The alloy of claim 1 including 0.003 to 0.10 in total of calcium and magnesium.

6. A heat-resisting weldable alloy consisting essentially of by weight percent, about 0.1 to 0.4 aluminum, 0.001 to 0.008 calcium, 0.002 to 0.1 magnesium, 0 to 0.08 carbon, 0 to 1 manganese, 0 to 0.002 sulfur, 0 to 1 silicon, 0 to 0.75 copper, 0 to 0.03 phosphorous, 18 to 22 nickel, 18 to 22 chromium, and the balance iron and incidental impurities.

7. The alloy of claim 6 including 0.15 to 0.25 aluminum.

8. The alloy of claim 6 including 0.002 to 0.006 calcium.

9. The alloy of claim 6 including 0.002 to 0.006 magnesium.

10. The alloy of claim 6 including 0.003 to 0.10 in total of calcium and magnesium.