CA1330628C - Low carbon plus nitrogen free-machining austenitic stainless steels with improved machinability and corrosion resistance - Google Patents

Abstract

ABSTRACT
A low carbon plus nitrogen, free-machining, austenitic stainless steel having improved machinability and excellent cor-rosion resistance. The steel composition in weight percent is carbon plus nitrogen up to about 0.06, chromium 16 to 20, nickel 6 to 14, manganese up to 0.60, sulfur 0.15 to 0.50, silicon up to about 1, phosphorus up to about 0.20, molybdenum up to about 1 and balance iron.

Description

~ 3 3 ~ & .2 3 ..
Conventional resulfurized free-machininq austenitic stain-less steels such as AISI Type 303 generally do not have suffi-cient corrosion resistance to allow them to be used in applica-tions for acid soft drink or beverage syrups withoutsignificantly affecting the flavor of these products. The prob-lem largely relates to the fact that the manganese or manganese-rich sulfides present in Type 303 are readily attacked - in acid ~oft drink or beverage syrups. As a result of this attack, the local environment is 80 changed that the stainless steel adjacent to the mangane~e or manganese-rich sulfides cor-rodes, thereby releasing both sulfide and metal ions into the syrUps and causing odor or ta~te problems. Passivating free-machiing austenitic stainless steels such as ~ISI Type 303 in nitric acid solutions can minimize this diffic~lty by removing most of the manganese or manganese-rich sulfides from the sur-faces of the articles machined from these steels before they are placed in service. However, the general corrosion resistance of the stainless steel matrix of AISI Type 303 i8 of~en insuffi-cient, even in the absence of substantial sulfide dissolution, toavoid changes in the quality or taste of the beverage syrups.
Thus, to improve the corrosion reæistance of free-machining austenitic stainless steels in acid beverage syrups, the use of a more corrosion resistant free-machining additive along with im-provements in the general corrosion resistance of the steel ma-trix in acid beverage syrups are necessary.
. r~ ;~
. -1-.
~, . ~
~ ~''~ .` ' ' ' ~ ' ~ :

3 ~
l In this respect, U.S. Patent 3,902,398 discloses that the corrosion resistance of resulfurized free-machining austenitic stainless steels can be significantly improved in acid beverage syrups by restricting their manganese content to a maximum of about 0.50% and by controlling the manganese to sulfur ratio such that chromium or chromium-rich sulfides are formed instead of manganese or manganese-rich sulfides. Chromium sulfides are more corrosion resistant than are manganese or manganese-rich sulfides in acid beverage syrups, and improve machinability but not nearly to the same extent as manganese or manganese-rich sulfides. AB
al~o disclosed in U.S. Patent 3,902,398, the loss in machinability related to the replacement of manganese or manganese-rich sulfides by chxomium sulfides can be partly offset by lowering the carbon conten1: of such steels to below about-lS 0.~35%.
In accordance with the pxesent invention, the machinabi~ityof low-carbon resulfurized austenitic stainless steels containing chromium or chromium-rich sulfides can be substantially improved :
- by controlling their carbon plus ni~rogen content to lower than conventional levels. It has further been discovered~that the addition of copper, which is known to improve the màchinability o~ other austenitic stainless steels, not only improves the machinability of these low-manganese free-machining austenitic stainless steels, but also significantly improves their corrosi~n resistance in acid soft drink and beverage syrups. Thus, it is , possible tc substantially impr~ve the machinability of . . ..

- ' :

1 free-machining austenitic stainless steels containing chromium or chromium-rich sulfides by lowering their carbon plus nitrogen content b~low conventional levels, and to further improve their machinability and especially their resistance to corrosion in acid soft drink or beverage syrups by increasing their copper . content within closely controlled limits.
OBJECTS OF T~E INVENTION
It is accordingly a primary ob~ect of the present invention to provide a chromium-nickel, free-machining austenitic stainless steel having improved machinability and high re~istance to corro-sion, especially in ac~d ~oft drink or beverage syrup~.
An additional object o~ the invention is to provide 2 chromium-nickel-copper bearing austenitic stainless havinq im-proved machinability and substantially better corrosion re~
tance, especially in acid soft drink and beverage syrup~.
Another object of this invention is to provide machined chromium-nickel austenitic stainless steel fittings and articles with improved machinability and high corrosion resistance,.espe-cially in acid soft drink and beverage syrups.
Yet another object of this invention is to provide machined chromium-nickel-copper au~enitic ~tainless steel fi~tings and articles with substantially improved machinability and excellent corrosion resistance, especially in acid soft drink and beverage ~yrups.
.

.

i :,.- ,' ~;`. '.- ~, ,:'' ~ ~ . .

. ~ . . , , ~

i~ 1 SUMMARY OF THE INVENTION
In accordance with this invention, it has been discovered that the machinability of chromium-nickel austenitic stainless steels containing chromium or chromium-rich sulfides and with low-manganese up to 0.50% can be greatly improved by reducing their carbon plus nitrogen contents below conventional levels.
In this regard, total car~on plus ni~rogen in combination at low levels in accordance with the invention is more effective than either low carbon or nitrogen alone. In addition, it has been discovered that the addition of copper to these steels in con-trolled amounts not only improves machinability, but more impor-tantly significantly improves their corrosion resistance, partic-i~
. ularly in the passivated condltion, in acid soft drink syrups.
The improvements in machinability achie~ed by reducing carbon plus nitrogen content are obtained both at residual and elevated copper contents. However, the greatest improvements in . machinability as well as in the resistance to corrosion in acid soft drinks are obtained with the copper bearing steels o~ this .~ invention.
The steels of this invention have particul~r ad~antage in the application of fittings and articles used for handling and di~pensing acid soft drin~ syrups. With these steels, the decrease in machinability normally associated with the repla~e-ment of manga~ese or manganese-rich sulfides by chromium or chromium-rich sulfides is offset by the lower than conventional oarbon plus nitrogen contents and by the additiion of copper.

__ -4-Further, the copper bearing steels of this invention exhibit much better corrosion resistance in acid soft drink syrups, which is an additional advantage over prior art steels used in these applications.
In their broad composition range, the steels and machined fittings and articles of this invention consist essentially of the following elements, by weight percent:
carbon plu5 nitrogen - up to about 0.05%
chromium - 16 to 20 nickel - 6 to 14 manganese - up to 0.60 sulfur - 0.15 to 0.50 phosphorus - up to 0.20~
silicon - up to 1~ -. 15 molybdenum - up to 1%
:~ iron - balance, except for incidental impurities Carbon and nitrogen are normally present in the steels of this invention, but to obtain the desired improvements in .~. machinability, it is essential in the steels of this invention to 20 control the total carbon plu8 nitrogen levels below about 0.06 ~ and preferably below about 0.05 or 0.04~.
-~ In general, about 16 to 20% chromium and preferably 17 to 19~ chromium is required in the steels of this invention to ~btain the required degree of corrosion resistance in acid soft ~ 25 drink syrups and to adjust for the amount of chromium involved in 'Js the formation of chromium or chromium-rich sulfides.
.
.

, . .
:

'Y` `:
.`~
~, .

:

-P i3~2~
Q'.

1 About 6 to 14~i and preferably 8 to 11% nickel are required in the steels of this invention to obtain an austenitic microstructure and to minimuze austenite transformation during processing operations at ambient temperature.
A maximum of about 0.60% manganese is required to minimize the formation of manganese or manganese-rich sulfides which are known to have an adverse effect on corrosion resistance in acid soft drink syrups and still permit the use of low cost scrap re-vert melting practices. In applications where maximum resistance ; to corrosion in acid soft drinks is required, the manganese con-tent must be controlled below about 0.50 and the maximum manga-nese to sulfur ratio is 1 to 1.
A minimum of about 0.15 and a maximum of about 0.50% of sul-fur are needed in the steels of this invention to obtain the desired degree of machinability.
Copper in amounts of about 0.75 to 3.00 and preferably in the amounts of 1.00 to 2.50 is very useful for increasing the stability of the austenite, for improving the machinability, and particularly for increasing the corrosion resistance of the i ~iteels of this invention in acid soft drink syrups.
~¦ Molybdenum is not necessary in the stels of this invention, but may be used in amounts up to about 1 perFent for improving general corrosion resistance.
Silicon and phosphorus, may be present in amounts up to ~5 about 1% and 0.20%, respectively, in the steels of this inven-tion. The remainder of the composition is esisentially irorl, i .
1 -6- j !
. . .
, ,~
.,. ~

~f . . . . .
1 ~ 3 l except ~or incidental impurities usually associated with the pro- -duction of stainless steels and except for up to 0.01% boron which may be added to improve hot workability.
DETAILED DESCRIPTION OF TH~ INVENTION AND
SPECIFIC EXAMPLES
To demonstrate the invention and specifically the heretofore undisclosed effect~ of low carbon plu3 nitrogen and copper in accordance with the invention on machinability and corrosion re-sistance, fourteen 50-pound laboratory heats were vacuum induc-tion melted and cast into ingots. The ingots were heated to 2200~F and hot forged to l-l/4-inch octagonal haped bars and air cooled. The bar~ in turn were annealed by heating to 1950F, holding at l950~F for one hour, ~nd then wster quenching. Sam-p~e~ from these bar~ were ~achined to o~e inch sguare by four inch long specimens for drill machinability testing.

..
' : , , ' , ~ .. .

'`'~
-. ~ ~ ~

-`? . ~ . -'` "''`'` l~a~2~
~ ~ u~ ~ O a~ ~ ~ ~ a~ ~r -~ ~ o o o o o o a~ o ,l o o ,~
. ~ Z ~ ~ ~ u~ r- I` O ~ ~r 1~: + oo oooo_10000000 ~ oo oooooooc~oooo ~ o ~ o o ~ ~ N t~ I N ~
;`.~ O Zloo ~loooocsoooooo ~c .a .. ............
00 00000000~00 .
. ~ . ~ ~ C~ ~ N N ~ N r~
U

a ~ ~ ~r~
~ ~ - .-----.-....... -. -~ ~_ oO oOoooooooooo '.3 P; O
~rl ~D O U- ~ N ~ C~l N C'~l ~ ~
I ~ul ~ o co OD ~ 0 C~ 00 Cl~ CO ~ ~ 0 . _~ ~
. 1~
Q
. I _ . l Zl oo 000000~00000 I ~
. 1 8 , "
l ~ oo ooooooooooo~
lo I ~1 ~ O N ~
l t,ql . ....... ~.... ~
1 00 C~ooooooOooOo I
O a~ O N ~1 o u~ IJ~ O) . o N ~ ~ C~ N ~ N ~ tr5 t~ ~ ~ ~ N
' . ~ P~l 00 000000000000 ~3 OC~ OOOOC:IOOOOOOO
~1 ~ ' ~

C~ r~ o ~ t~ ~ o u) ~
. ~ - 00 000000000000 ~ ..
.1 ' . ~ ~ ~3 ~ O ~- O _I CO O ~ ~
C.)l 00 000000000000 OO OoOOOOOOooC~O
. ~ , .
h : ' ~ ~ 0 ~ ~ o o o o o ~ ~ o o I Z ~ p ;~

~3~$,~

1 Table I lists the resulting chemical compositions of the laboratory ~eats. Other than variations in carbon, nitrogen, mangane~e, molybdenum and copper, all the alloys are essentially 0.40 percent sulfur, 18 percent chromium, 10 percent nickel, 5 free-machining austenitic stainless steels.
~ he machinability of the experimental alloys-of Table I was evaluated using the aforementioned test specimen and a drill machinabilty test. In the drill machinability test, the total time taken to drill a ~pecified number of holes to a speclfied depth in the material to be evaluated i5 compared to the total time to drill the same number of holes to the same.~depth in a material having known, establlshed machining characteristics.
The ratio between the time taken to drill ths established mate-rial and the time taken to drill the-test material multiplied by 100 provides a drill machinab:Llity rating (DMR) for ~he test material. Specific condition~ used for these tests were as fol-lows:
Drills - 1/4 inch diameter high speed steel ~obber bit~
Drill Speed - 405 revolutions per minute Load on Driil - 14.2 pound~
. Break-in ~ole Depth - 0.1 inch ~imed ~ole Depth - 0.3 inch Drill Machinability , Time to drill established steel X 100 Rating (DMR) Time to.drill test steel Reat number V506 containing 0.079 percent carbon plus nitro--'S gen, about the concentrations of these elements in a typical ~9~

s~ ~
~;

1 steel of this type, was assigned a DMR of 100. Thus, steels :~! having DMR values of greater than 100 have better drill machinability than conventional, typical steels of this type; and values less than 100, poorer drill machinability. Also, increas-S ing DMR values indicate improved drill machina~ility.
$able I presents the results of one drill machinability t-sting of the laboratory steels. Allowing for some experimental scatter in the data and considering the steel~ containing about 0.30 percent copper and 0.025 to 0.106 percent carbon plus nitro-gen, i.e., heat number V489, V505, V560, V603, V603A, V506, and ~ V541, it is clearly evident that lowering the total combination.~
of carbon plus nitrogen content of the steel results in improved drill machinability. Steels within the scope of the invention, i.e., heat number V489, V505, V560, and V603, ail display im-proved machinability compared to heat number V506.
- The data in Table I also show that heat numbers V560 and V603, which have similar carbon plus nitrogen contents of about 0.05~, have similar drill machinability despite the fact that ~he carbon contents of the heats are respectively below and above the critical value of 0.035% specified in U.S. Patent 3,9Q2,398 ~or stainless steels of this type. A like result is obtained when comparing the drill machinability of heats V506 and V603A, which have similar carbon plus nitrogen contents of about 0.075~ and ~carbon contents respecti~ely below and above the critical value of 0.035~ specified in the above patent. ~hus, carbon plus . ., :~

`~ 3 ~

~ 1 nitroQen conten~ is more critical than carbon csntent in regard :~, to the machinability of the low man~anese austenitic stainless }i :3~ steels of this invention.
At an equivalent carbon plus nitrogen content, adding at least 1.24 percent copper to the invention steels result~ in still further improvemets in machinability as illustrated by heat numbers V508, ~507, V564, V567, and V565. A molybdenum addition to heat number V568 appear~ to have essentially no effect on drill machinability when compared to heat number V567 containi~g a similar amount of copper but less molybdenum and slightly less carbon plu~ nitrogen.
An empirical test in a commercial acid soft drink syrup sold under the registered tradename SPRITE was conducted to compare the corrosion resistance of heat numbers V505, VS06, V562, V507, V508, V564, and V565 with those disclosed in U.S. Patent J

3,902,348, and AISI Type 303 stainless. In this test, six-inch lengths of the bars produced from these stainless steels and the -AISI Type 303 were miiled to the bar centers in order to obtain chip~ that were representative of the entire bar cross section.
Ten grams o~ both as-machined and passivated ~50% nitric acid . ~ .
plu5 2% sodium dichromate) chips were then immersed in 50 milllliters lml) of SPRITE syrup (pH-3) for five days. During this period of exposure, hydrogen sulfide (~ S) gas generation was monitored with moistened lead acetate teQt paper. The color change.s.in the lead acetate paper, if any, were xeco~ded and .
. . ..
~ ~.- ~;
.
.

. .

1 3 ~ 3 ~ ~ 8 . ., ''? 1 rated visually in regard to the degree of H S evolution according i~ to the following system: 0 - None, 1 - very light, 2 - light, 3 moderate, 4 - heavy, 5 - very heavy. Also, at the end of the five day test period, the syrups were s~parated from the chips S and diluted to 200 ml with deionized water. The dilute syrups were then analyzed for iron, manganese, nickel, chromium and cop-per ions. The results of all the soft drink syrup tests are given in Table II.

. ' , .
.-' , . .. ~ . , -, ~ ~ . -` ~

~ ~ : .-:

3 ~
; ~

o .~ ~ I P ' ~ wo ~ ~
_I oo ~ u~u~u~r~ ~ ~
~ ~ ~ ~ O~ l ~ O
~; O c~ ls, )I ~ .
~ ~ ~ ~ 3 ~.- ~ ~ ,~ a3 ~ _~ ~ ~ u o ~
~o~ . ~ ~ ~ WU~ o ~1 ,~n ~
~ _ N
i~ 1~ . ~ .
., ~ ~
~V ~:
00 ~ t: '~O . O O O O O O C~
Q~ 1: ~a t.~ ~ P~
roP; . ~, . . #1~ ~ ~ ~ I U~
.
.C 8~P S
i~ ~ ~ Ooooooooln ' C ~
al ~ ~ ~o ~ o ' ~ ~¢ ~_ I I O
. ~ . . - o .~ ~ ~ o~ ~ U O
. C ~ ~N 1` t~`J N 0 `~, tl~ O O O ~ O ~1 0 C
: O rl O a ~1 ~ ~ ~ ~ a~ 0 ~
:-~ ~0 ~ ~0 ~1 0 ' E~ :ZI ooooo oo~ ~
.- I ~ ~ o w ~
. : ~ ~ o ~
~ ~ 1 t,q~
. ~ O oooooooo .~ ~ ~
~ @1 6 . . O oo~ o~o E~~o~,o . . u 51 ~ ~ ~ r' ~ ~ H
: ;~
: 0000000 :. . . 10~ ~ _~
'.'7 Z ~ ~ U
.i~ + oooooooo .. j, u ........ " ~ a~
'i 0 0 0 0 0 0 0 0 , ~
.~i ~ E~ O O U~ O O ~
~ . P Z ~ '7 ~
~' ..
'~'~'`:'.. ' ' ' i ~

'.;, :' .'.~ : ' . . ' ' ' . , ' ~, . ,:

3 J i~ f~ ~

1 The results of the lead acetate paper monitoring of H S gen-eration and of the syrup analyses indicate that increasing ~he copper content of the low manganese-chromium-nickel, free-machining stainless steels of this invention to above about 0.75% and particularly from 1.26 to 2.29~ siqnificantly improves their resistance to corrosion in SPRITE soft drink syrup, espe-cially in the passivated condition. This effect of copper is most clearly evidence by heat numbers V507, V564, and V565 which contain 1.26, 1.79, and 2.29% copper, respectively, and which show essentially no H S evolution during testing and signifi-cantly less contamination of the SPRITE soft drink syrup than do similar steels with residual copper, such as ~eat V506, and much less than AISI Type 303, as represented by heat number A-15596.
Thus, the low carbon plus nitrogen, low manganese, copper bearing austenitic stainless steels of this invention exhibit much better resistance to corrosion in acid soft drink syrups than do prior art steels of this general type.

. ~ , . .. .
' ' ' . .

. -14-:
,

Claims (16)

1. A low carbon plus nitrogen, free-machining, austenitic stainless steel having improved machinability and excellent resistance to corrosion in acid soft drink syrups, especially in the passivated condition, said steel consisting essentially of, by weight percent carbon plus nitrogen up to about 0.05 chromium 16 to 20 nickel 6 to 14 manganese up to 0.60 sulfur 0.15 to 0.50 silicon up to about 1 phosphorus up to about 0.20 molybdenum up to about 1.0 and remainder iron except for incidental impurities.

2. The steel of claim 1 having carbon plus nitrogen up to about 0.04.

3. The steel of claim 1 or 2 having from about 0.75 to 3.00 copper.

4. The steel of claim 1 or 2 having 1.00 to 2.50 copper.

5. A low carbon plus nitrogen, free-machining, austenitic stainless steel having improved machinability and excellent resistance to corrosion in acid soft drink syrups, especially in the passivated condition, said steel consisting essentially of, by weight percent carbon plus nitrogen up to about 0.05 chromium 17 to 19 nickel 8 to 11 manganese up to 0.50 sulfur 0.15 to 0.50 silicon up to about 1 phosphorus up to about 0.20 and remainder iron except for incidental impurities and wherein the maximum manganese to sulfur ratio is 1 to 1.

6. The steel of claim 5 having carbon plus nitrogen up to about 0.04.

7. The steel of claim 5 or 6 having from about 0.75 to 3.00 copper.

8. The steel of claim 5 or 6 having from 1.00 to 2.50 copper.

9. Machined austenitic stainless steel fittings and articles characterized by having improved machinability and resistance to corrosion in acid soft drink syrups, especially in the passivated condition, said fittings and articles consisting essentially of, by weight percent carbon plus nitrogen up to about 0.05 chromium 16 to 20 nickel 6 to 14 manganese up to 0.60 sulfur 0.15 to 0.50 silicon up to about 1 phosphorus up to about 0.20 molybdenum up to about 1 and remainder iron except for incidental impurities.

10. The machined austenitic stainless steel fittings and articles of claim 9 having carbon plus nitrogen up to about 0.04.

11. The machined austenitic stainless steel fittings and articles of claim 9 or 10 having from about 0.75 to 3.00 copper.

12. The machined austenitic stainless steel fittings and articles of claim 9 or 10 having from 1.00 to 2.50 copper.

13. Machined austenitic stainless steel fittings and articles characterized by having improved machinability and improved resistance to corrosion in acid soft drink syrups especially in the passivated condition, said fittings and articles consisting essentially of, by weight percent carbon plus nitrogen up to about 0.05 chromium 17 to 19 nickel 8 to 11 manganese up to 0.50 sulfur 0.15 to 0.50 silicon up to about 1 phosphorus up to about 0.20 molybdenum up to 1 and remainder iron except for incidential impurities and wherein the maximum manganese to sulfur ratio is 1 to 1.

14. The machined austenitic stainless steel fittings and articles of claim 13 having carbon plus nitrogen up to about 0.04.

15. The machined austenitic stainless steel fittings and articles of claim 13 or 14 having from about 0.75 to 3.00 copper.

16. The machined austenitic stainless steel fittings and articles of claim 13 or 14 having 1.00 to 2.50 copper.