CA1205692A - Diffusion coating and products - Google Patents

Abstract

DIFFUSION COATING AND PRODUCTS

Abstract Low alloy steels are made resistant to hot sulfidation by first decarburizing then chromizing then aluminizing, either in independent operations or run together in a single retort.

Description

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DIFFVSl~N COATING AND PRODUCTS

The present invent~on relates tG the coating of metals t~ inc_ease their resistance t~ corrosion and o~her chemical ateack6.
Among the objects o the present invention is the provision of novel coatin~ techniques and compositions for use therewith, as well as nov~l coated produ~ts, all suieable for ~ ~ cial operations.
Additional objects of the present invent~o~
include techniques for conf~nin~ protPctive diffusion coatin~s to desired locations on workpieces.
The foregoing as well as further objects of the present invention will be more fully understood from the following description of several of its .
exemplifications.
- For increasing the resistance of steels, e~en low alloy steels, to attack by hot sulfur-containing ~aterials, chromizin~ is very effective when it pene-trates more than sbout 3 mils and is followed`by a~
even deeper aluminizing. ~ess penetrating treatm~nt of~superalloys i6 disclosed in U. S. Patents 3,?64,371, 3,694,255 and 4,041,196, a~d a s~milar treatment 1~

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`- :lZ-~5~:;92 disclosed in U.S. Patent 3,656,919, but the present invention applies such a sequence to low alloy steels such as those called chromium steels. These chromium steels of the present invention contain from about 0.6 to about 1.5~ chromium, and their carbon contents can range from about 0.15% to about 1.10%. This carbon content is not in stabilized form and is accordingly sufficient to seriously limit the case depth obtained by chromizing, as ~ell as correspondingly limit the maximum resistance obtainable against hot sulfidation.
Unprotected low alloy steels used in petroleum refinery operations for example, are generally very susceptible to hot sulfidation, so that a high degree of protection is called for.
According to the present invention there is provided a process of protecting against hot sulfidation, low alloy steel equipment that is to ~e welded in plaGe. Thè
process includes the steps of subjecting the equipment to a heavy diffusion chromizing followed b~- a diffusion aluminizing in which the aluminizing is arranged to leave the welding sites with a surface aluminum content less than about 10~.
According to the one aspect of the present invention low alloy steels such as the above-noted chromium steels have their surfaces first decarburized to a depth of at least about 3 mils~ then chromized to provide a case at least about 2 mils thick, and then aluminized to a depth of at least about 6 mils. One preferred decarburizing is -to a depth of 4 to 6 mils, along with chromizing to provide a case thickness of about 3 to 4 mils, and aluminizing to a final difEusion case of about 10 to about 14 mils.

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Novel features and advantages oE the present ilvention in addi-tion to -those mentioned above, will become apparent to those skilled in the ar-t fro~ a reading of the following detailed description in conjunc-tion with the accompanying drawings wherein:
Fig. 1 can be described as a vertical sectional view showing one working arrangement for applying the difEusion coa-tings of the subject invention.
Fig. 2 can be described as a similar view oE
a modified working arrangemen-t.

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The respec~ive treatments can be applied individually in spaced operations, or they can be com-bined as successive stages of a single heat sequence.
Each stage can be a conventional one such as:
EXAMPLE I
. 1. A number of chromium steel tubes used fos a heat exchanger in the oxidative cracking of sulfur-contAining petroleum residues were cleaned, and then placPd in a retort through which hydrogen having a de~
point of 40 to 60F is pa~sed, while the reto~t ls heated to 1800-1850F for three hours. Before the treatment, the tubes had 0.7 to 0.8% chromium and 0.18%
carbon, their side walls were about l/4 inch thick, their internal diameter about 2 incnesJ and one end of each tube was flared out and mushroomed back. After the three hour treatment, heating was discontinued and the tubes permitted to cool to room temperature. They showed a surface decarburization about 6 mils deep.
2. The decarburizeB tubes ~ere then loaded i~
a vertical retort, the floor of which had a four-inch thick layer of powdered chr. z~nR pack lnto which the flared ends were em~edded. The interior of the tubes were filled with the same chromizing pack, which was a pre-fired 1:4 by weight mixture of chromium powder-ant the fluent tabular A1203 described in U.S.Patent O
4,208,453, with 112% NH4Cl based on the weight of th~
1:4 mixture. The retort was then covered and fired us in U.S.Patent 3,801,357 to bring its contents to 1950-~ .~, :` ; i .

~Z~6g2 ' 1975F for ten hours. Upon subsequent cool down the tubes showed a cbromized case 3 to 4 mils thick, the outer qu~rter havin~ 8 chromium-carbide-ricb phase with the remainder having a columnar ferrite structure.

3. The chromized tubes were returned to the vertical retort, which this time contained a layer o~
aluminizin~ powder pack, and the ~ube interiors were filled with the same aluminizing pack - a pre-fired mixture of 45 weight % chromium powder, 1~ weight %
aluminum powder and 45 weight % powdered alumina, activated with 112Z NH4Cl. The al~;ni7-ing heat w~s at 1950-1975~F for ten hourc and left a consolidated diffusion case 12 to 14 mils thick. The outer surface of this case contained about 21% aluminum and abo~t ~, 21L chromium, whereas the middle of the case con- -tained about 12.5% aluminum and about 13.7~ chrom~um.
These tubes showed ~ very high resistance against hot sulfidation, and in one instance a tube was attacked so severely $hat the steel base was completely e&ten through leaving an almost unscathed diffusion case as a shell.
A typical tube o the foregoing type is illustrated at 10 in Fig. 1, where it is shown held ln a retort 12 with its ~eversely flared end 14 ~mbedded ~n one of the diffusion coatln~ packs. Where the tube ~; ~

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has an extern~l surface portion that is not to sece$ve a dlffusiDn coating, as fos instance because that por-tion ~s closely fitted into a mouneing sheet or the like, that surface portion can be masked~ Thus a slurry type powdered alumina masking coating can be aDplied 8S illustrated at 18 and 20~ Alternatively a powder typè masking mixture can be poured into the cup-shaped portion of flare 14, and if deslsed such po~der can be held in place with the help or a steel ring fitted around the powder and resting on the up-turned lip of the flare.
The decarbsrizing can be conducted in Any con-venient way, although it is preferred tD use hyd~ogen having a dew point at least as high as 0~. The deca~-burizing temperature can vary in the manner shown in ~he pr~or art, and effective decarburizing produces a weigbe loss of from about 0.3 to about 0.6 milligrams per squ~re centimeter of treated surface. This can be checked by including in ~he retort a spare coupon of the metal being treated and withdrawing the coupon to check its weight.
The diffusion chromizing and sluminizing can also be varied as disclosed in the prior art. A good chro~-~um pick-up is at least 15 and bettes still over 25 milligrams per square centimeeer of surface, and a good aluminum p~ck-up i9 ae least 25 and preferably over 40 milligrams per square centimeter. Another gOoa combi-naeion has a pic~-up of over 30 milligrams of chrom~um ! ' 569~
~er square centimeter and a pick-up of about 20 to about 40 milligrams per square centimeter.
Although the aluminum pick-up appears large when measured in milligrams per square centimeter, the aluminum penetration is so deep and agressive that the aluminum concentration at the surface can be as little as 5~ by weight.
The aluminizing also causes the chromium diffuslon to deepen, but the final concentration of chromium of the surface is generally at least about 15%.
The separate treatments can be partially or completely run together in a single retort without removing the tubes from the retort between treatment steps. Some combinations of decarburizing with chromizing are mentioned in U.S. Patent 3,449,159, but the decarburizing of the present invention is a very substantial one in which the decarburized zone is not only deep but shows a carbon content well below 0.05~.
Gas chromizing as in U.S. Pater.t 3,449,159 and 3,222,122 is particularly desirable when consolidating the chromizing with both the decarburizing and the aluminizing.
The only pack in contact with the tubes in such a consolidation is the aluminizing pack, inasmuch as gas aluminizing is not sufficiently effective.
Fig. 2 illustrates a retorting arrangement for consolidated treatments. Two tubes 50 and 60 are here shown lowered through a perforated removable shelf 22 resting on the upper edge of a cylindrical retort shell which has welded around its upper margin a rela-cw/ ~ - 6 -`~

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tively short retort extension 28. The top o~ ehe retort is loDsely covered with a plate 30 fitted w~th a vent pipe 31 and a set of nozzles 32 downwardly directed over each eube-receiving perforation in shelf 22. For decarburizing, the tubes are mounted as at 50 and wet hydrogen ineroduced through nozzles 3~ so as to pass downwardly through and decarbur~ze ~he interiors of the tubes. Some of the introduced hydrogen al~o spills over the flared tube ends and decarburizes them as well.
When the decarburizing is completed the flow of wet hydrogen is stopped, and gas cnr~ zing streams are introduced through nozzles 32. ~o other change except for a shift to the chromizing temperature is needed, so tbat the retort can be kept at operating temperature~ throu~hout.
The next shift i~ to al~ n;7;ng and for this the cover 3Q or the nozzles 32 are removed so that a powder aluminizing pack can be poured ineo ehe tubes.
By keeping the lower ends of the tubes within an inch or so of the retort floor a low fluency pack can be poured in place without building up very much on that floor. ~n the other hand a more fluent pack can be used with more of the pack spreading out on the flocr as well as over the flared tube ends.
It will generally be desirable to cool the retort and ~ts contents somewhat before introducing the aluminizing pack. To this end the burners around -~ ~ ) .~ ~
-8 ~ 6~2 the retort can be shut down and g rapid flow of argon introduced through ven~ 31. Because no thermal insula-tion is present in the retort, aside ~rom the ~ask~n~
layers, the tubes sre fairly rapidly cooled in thls way. After sufficient argon flow to flush and ill the retort wlth tha~ gas, the retort cover or nozzles 32 can generally be removed even though the tubes are 8~ill at ~bout 900nF, especially if the rapid ar~on flow is maintained as a protective shield around the tubes and the openings at the retort top kept very small. The aluminizing pack can then be promptly poured through such openings into the tu~es to further cool them both by contact as well as volatilization o~
the acti~ator contained in the pack. To avoid pre-; mature depletion of the activator ~hen so added, $he activator content of the pack can be somewhat elevated, such as 1 11270 by weight, instead of the usual 114 to 1/2% by weight.
As soon as the pack addition is completed the ` ` retort-heating burners can be re-started to carry the retort contents to the desired aluminizing temperatur~.
Where the aluminizing pack is a chromium-free pack, ehe aluminizing can be very effectively conducted at tem-peratures as low as 1400~F or even lower, wlthout much reduction in c`oating rate. Aluminizing at temperature8 below sbout 1200aF ls best ~ffected with an aluminum P, ~' ) _g_ ~OS6~2 halide activator such as AlC13.
The cool~ng applied before the al, niY.~g can be to very low temperatures if desired. Thus coolin~ to about 6~0F will enable the introduction of an NH4Cl-activated pack without significant loss of N~4Cl by volatilization, and cooling to ~bout 350F will similarly ~im~7e loss of AlC13 f~om an AlC13-acti~ated pack. Cooling to 150~F will enable withdr~wal of the cbromized tubes from the setost, ~as for example if they are to be closely inspected to check on the chromized case or to replace the -Ckin~. -Cu~ of short pieces of tubin~ can be placed on shelf 22 to act as test membess that can be removed when desired to check on the treatment without remov$n~
the tubes themselves. ~' The consolidat`ing of the al~1miniz;nE with the previous treatmenes requires much more than the mere consolidation of the decarburizin~ with the chr~ '7ing.
This simpler consolidation can also be effected with the use of a chromizing pack rather than a gas phase chromizing, so that such pack is seplaced by an aiumi-nizing pack for the aluminizing step.
The exteriors of the tubes sub~ected to the three-step treatment of decarburizing, chromizing and-alumin$zing, also show the effects of such treatment steps, although these effects are 'ni~ized at the . --ir.

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iZ~ki92 ~asked areas, The decarburizing of ehe exteriors reduces the tube strength6, but this e~fect 13 insignificant when the tube wall thicknesses are greater than 1/8 inch. Also such decarburixed exter-iors become chromized to some degree and this stren~-thene those surfaces.
Other chromium steels snd other low ~lloy steels can be treated in place of the foregoing eubes, to give similarly protected products. Plain steels having about 0.1% or less of carbon are generally not s~rong enough or sufficiently resistant for use in hot environments, but they too can be similarly chrom-ized and aluminized to yield p~oduots showing ve~y little corrosion in hot sulfidation environments, und 0, do not need a preliminary decarburizing. Steels that have their carbon contents s~abilized as noted in U. S. Paten~ 3,449,159, l~kewise need no decarburizing.
A high degree of decarburizing is desirable for unstabilized carbon contents, in~ ch as this enablee very heavy diffusion chromizing. The following is an exemplification.
EXAMPLE II
~ he procedure of Example I is followed, with the ~ollowing rh~ngPs A. The decarb~rizing is conducted at 1825 to 1875~F for six hours with wet hydrogen havin~ a dew point ~t least a5 high ~s 20F to yiPld ~ decarburized sur~ace B to lO-m$~s deep.

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~2~)5~2 B. The chromizing is conducted with a ~on-prefired pack of 20% chromium and 80% non-~luent alumina, activated as in Exa~ple I. The ch~ ing ~s st 205~ to 2100F for 10 hours, leaving a chr~ '7ed case 11 to 12 mils deep essentially all columnar ferrite and with little or no distinguishable chromium-carbide-rich phase. It is noted that the chromizing causes a little increase in the decarburizin~ dept~ and is as deep as the resulting decarburizing. The surface chromium content is about 50 milligrams per square centimeter. The chromizing also causes significant loss of carbon from the body of the substrate.
C. The aluminizing is conducted with a non-prefired pack of, by weiRht:
5.4% aluminum 46.4% chromium balance alumina activated as in Example I. The alumini~ing temperature ~8 2050 to 2100F for 10 hours. The resultin case is about 20 mils deep and the aluminum concentration at the surface is about 12%.
~ bdi~yin~ E~,ple II by reducing the ~ - content of the Pl~rini7in~ pack to 2.5% and its ~- - to 43.5% yields a c~ce depth of about 18 mils and a surface aluninun~content of abcut 6%.
me pnxhcts of Example II sb~ a so~at better resist-ance than that of Example 1, to oxidative attack by air at 1500F.
Theuasking ~ nnc used in the process of the present invention are preferably free of metallic alumiaum or metallic aluminides. Alumin~ alone is ~n j, ;

-12_ lZ~)5~2 effective maskant for the techniques of the e~a~ples, ln~ h as the masking is noe in a location sontacted by a diffusion coating pack.
On ordinary irons and steels ss well as low slloy steels, a localized layer of powdered ~ron sppropristely diluted with inert diluent 6uch ss slumina, csn be applied to reduce or prevent diffu6~0n coating under the lsyer. This lsyer csn be covered by a sheath-forming layer, but such covering is ~ot needed where the workpieces sre not roughly hsndled during trestment.
Powdered iron containing chromium can alss be used, suitably diluted, particulsrly to mask chromium steels against all~in;Y;ng. Nickel can slso be present in such 8 masking powder, even though it tends to diffuse into the masked surface. It is generslly mose important to keep aluminum from diffusing into the surfaces to be masked, in~ Ich as the sl~minizing is so aggressive and leaves a surface difficult to weld.
It tskes st least about 207. diluent powder of non-me~allic masking composition to the point thst it does no~ sinter to the workpiece.
Where the presence of a little alu~inu~ can be tolerated, the mask~ng compositions of U. S. Patent

4~208,453 can be used.
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lZ05692 Obvio~ly many ~odifications and variat~ons o~
the present invention are possible in the light ~f the above teachings. It is therefore to be understood thst within the scope of the appended clalms the inven-tion may be practiced otherwise than as specifically tescribed.
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Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The process of protecting against hot sulfidation a low alloy steel equipment that is to be welded in place, which process is characterized by the steps of subjecting the equipment to a heavy diffusion chromizing, followed by a diffusion aluminizing in which the aluminizing is arranged to leave the welding sites with a surface aluminum content less than about 10%.

2. The process of claim 1 in which the chromizing is sufficiently heavy to leave the aluminized surface with a chromium content of at least about 25%.

3. The process of claim 1 in which the outermost aluminum content is about 5% and is obtained without blocking the aluminizing.

4. The process of claim 1 in which the low alloy steel is a chromium steel.

5. The process of claim 4 in which the chromium steel has its surface decarburized before it is subjected to the diffusion chromizing.

6. The product produced by the process of claim 1.

7. The process of increasing the resistance to high-temperature attack by sulfur-containing petroleum ingredients of low alloy carbon steel refinery equipment, which process is characterized by decarburizing to a depth of at least about 3 mils the equipment face exposed to the attack, diffusion chromizing the decarburized layer to produce a chromized case at least about 2 mils thick the inner portion of which has a columnar ferrite structure, and diffusion aluminizing the chromized case to provide a final diffusion case at least 10 mils thick and thicker than the chromized case.

8. The process of claim 7 in which the steel is a chromium steel containing carbon in an unstabilized condition, the decarburizing is to a depth of at least about 4 mils, and the chromized case has a thickness of at least about 3 mils.

The process of claim 7 in which the equipment is a tube and the diffusion coating is applied essentially only to the surface of the tube exposed to the attack.

10. The process of claim 9 in which mounting portions on the surface of the tube are masked to minimize coating formation in those locations.

11. The process of claim 9 in which the equipment is a tube, the diffusion coating is effected with a coating pack in contact with said tube surface, and the adjacent surfaces of the tube are covered by inert diluent during the diffusion.

12. The product produced by the process of claim 7.

13. The process of claim 11 in which the outer surface of an end of the tube is covered with the inert diluent and that end of the tube is mushroomed back on itself.