TITANIUM-FREE ALLOY

Abstract

Titanium-free alloy which has great resistance to pitting and crevice corrosion and a high yield point in the strain-hardened state and includes (in wt %) a maximum of 0.02% C, a maximum of 0.01% S, a maximum of 0.03% N, 20.0-23.0% Cr, 39.0-44.0% Ni, 0.4-<1.0% Mn, 0.1-<0.5% Si, >4.0-<7.0% Mo, a maximum of 0.15% Nb, >1.5-<2.5% Cu, 0.05-<0.3% Al, a maximum of 0.5% Co, 0.001-<0.005% B, 0.005-<0.015% Mg, the remainder consisting of Fe and smelting-related impurities.

Description

The invention relates to a titanium-free alloy with high pitting and crevice corrosion resistance as well as high offset yield strength and tensile strength in the cold-worked condition.

The high-corrosion-resistant material Alloy 825 is used for critical applications in the chemical industry and in the offshore technology. It is marketed under the material number 2.4858 and has the following chemical composition: C≦0.025%, S≦0.015%, Cr 19.5-23.5%, Ni 28-46%, Mn≦1%, Si≦0.5%, Mo 2.5-3.5%, Ti 0.6-1.2%, Cu 1.5-3%, Al≦0.2%, Co≦1%, Fe the rest.

For new applications in the oil and gas industry, the pitting and crevice corrosion resistance (problem 1) as well as the offset yield strength and tensile strength (problem 2) are too low.

As regards the low chromium and molybdenum content, Alloy 825 has only a relatively low effective sum (PRE=1×% Cr+3.3×% Mo). By the effective sum PRE, the person skilled in the art understands the Pitting Resistance Equivalent.

The alloy that is Alloy 825 is a titanium-stabilized alloy. However, titanium may lead to problems, especially in continuous casting, since it reacts with the SiO₂ of the casting powder (problem 3). It would be desirable to avoid the element titanium, but that would lead to a significant increase of the edge-cracking tendency.

JP 61288041 A1 relates to an alloy of the following composition: C<0.045%, S<0.03%, N 0.005-0.2%, Cr 14-26%, Mn<1%, Si<1%, Mo<8%, Cu<2%, Fe<25%, Al<2%, B 0.001-0.1%, Mg 0.005-0.5%, the rest Ni. The content of Nb is generated by a formula. Furthermore, at least one of the elements Ti, Al, Zr, W, Ta, V, Hf may be present in contents≦2.

U.S. Pat. No. 2,777,766 discloses an alloy of the following composition: C<0.25%, Cr 18-25%, Ni 35-50%, Mo 2-12%, Nb 0.1-5%, Cu up to 2.5%, W up to 5%, Fe the rest (min. 15%).

The task of the invention is to provide an alloy alternative to Alloy 825 that remedies the problems outlined above and

• • is titanium-free,
  • has a high pitting and crevice corrosion resistance,
  • has a higher offset yield strength in the cold-worked condition,
  • has at least equally good hot formability and weldability.

Furthermore, a process for manufacture of the alloy will be presented.

This task is accomplished by a titanium-free alloy with high pitting corrosion resistance with (in wt %)

C  max. 0.02%
S  max. 0.01%
N  max. 0.03%
Cr 20.0-23.0%
Ni 39.0-44.0%
Mn 0.4-<1.0%
Si 0.1-<0.5%
Mo >4.0-<7.0%
Nb max. 0.15%
Cu >1.5-<2.5%
Al 0.05-<0.3%
Co max. 0.5%
B  0.001-<0.005%
Mg 0.005-<0.015%
Fe the rest as well as melting-related impurities.

Advantageous improvements of the alloy according to the invention can be inferred from the associated objective dependent claims.

An expedient embodiment of the alloy according to the invention has the following composition (in wt %)

C  max. 0.015%
S  max. 0.005%
N  max. 0.02%
Cr 21.0-<23%
Ni >39.0-<43.0%
Mn 0.5-0.9%
Si 0.2-<0.5%
Mo >4.5-6.5%
Nb max. 0.15%
Cu >1.6-<2.3%
Al 0.06-<0.25%
Co max. 0.5%
B  0.002-0.004%
Mg 0.006-0.015%
Fe the rest as well as melting-related impurities.

The content of chromium may be further modified if necessary as follows:

Cr >21.5-<23%
Cr 22.0-<23%

The nickel content may be further modified if necessary as follows:

Ni >39.0-<42%
Ni >39.0-<41%

The molybdenum content may be further modified if necessary as follows:

Mo >5-<6.5%
Mo >5-<6.2%

The content of copper may be further adjusted if necessary as follows:

Cu >1.6-<2.0%

If necessary, the element V may also be added to the alloy in contents (in wt %) of

V >0-1.0%
V 0.2-0.7%

The iron content in the alloy according to the invention should be >22%.

If the element titanium is left out, then—as explained above—edge cracks develop during rolling. The cracking tendency can be positively influenced by magnesium on the order of 50-150 ppm. The associated/investigated laboratory heats are listed in Table 1.

TABLE 1
Influence of deoxidizing elements on the edge-cracking tendency
during hot rolling
Element															Mg	Ca
in															in	in	Edge
wt %	C	S	N	Cr	Ni	Mn	Si	Mo	Ti	Nb	Cu	Fe	Al	B	ppm	ppm	cracks
Ref 825	0.002	0.0048	0.006	22.25	39.41	0.8	0.3	3.27	0.8	0.01	2	R	0.14	0	—	—	no
LB2181	0.002	0.004	0.006	22.57	39.76	0.8	0.3	3.27	0.4	0.01	2.1	R	0.12	0	—	—	slight
LB2182	0.006	0.003	0.052>	22.46	39.71	0.8	0.3	3.27	—	0.01	2	R	0.11	0	—	—	yes
LB2183	0.002	0.004	0.094>	22.65	39.61	0.8	0.3	3.28	—	0.01	1.9	R	0.1	0	—	—	yes
LB2218	0.005	0.0031	0.048>	22.50	39.59	0.8	0.3	3.27	—	0.01	2	R	0.12	0.01	100	—	no
LB2219	0.005	0.0021	0.043>	22.71	39.99	0.8	0.3	4.00>	—	0.01	2	R	0.10	0.01	100	—	no
LB2220	0.004	0.00202	0.042>	22.66	39.64	0.8	0.33	4.93>	—	0.01	2	R	0.11	0	100	—	no
LB2221	0.004	0.0022	0.038>	22.43	39.66	0.8	0.3	3.74>	—	0.01	1.9	R	0.11	0	10	—	yes
LB2222	0.003	0.0033	0.042>	22.5	39.62	0.8	0.3	3.66>	—	0.01	2	R	0.18	0	20	—	yes
LB2223	0.002	0.0036	0.041>	22.4	39.78	0.7	0.3	3.65>	—	0.01	2.00	R	0.27>	0	20	—	yes
LB2234	0.003	0.005	0.007	22.57	39.77	0.8	0.3	3.26	—	0.01	2.1	R	0.15	0	80	10	no
LB2235	0.003	0.0034	0.006	22.56	39.67	0.8	0.3	3.28	—	0.01	2.1	R	0.12	0	150	12	no
LB2236	0.002	0.004	0.006	22.34	39.46	0.8	0.3	3.27	—	0.01	2	R	0.11	0	30	42	slight
LB2317	0.001	0.0025	0.030	22.48	40.09	0.8	0.3	4.21	—	0.01(	2	R	0.16	0	100	5	no
LB2318	0.002	0.0036	0.038>	22.76	39.77	0.8	0.3	5.20>	—	0.01	2.1	R	0.15	0	100	4	no
LB2319	0.002(	0.0039	0.043>	22.93>	39.79	0.8	0.3	6.06	—	0.01	2.2	R	0.12	0	100	3	no
LB2321	0.002	0.0051	0.040>	22.56	40.23>	0.7	0.3	6.23	—	0.01	2.1	R	0.10	0	100	4	no

The effective sum PRE in regard to the corrosion resistance of the Alloy 825 is equal to PRE 33 and is very low compared with other alloys. Table 2 shows the effective sums PRE according to the prior art.

TABLE 2
Effective sum PRE for various alloys corresponding to the prior art
Alloy	Ni	Fe	Cr	Mo	Others	PRE
Duplex 2205	5.5	Rest	22	3	0.15 N	37
825	40	31	23	3.2		33
28	31	35	27	3.5	1.3 Cu	38
926	25	Rest	19	6	0.16 N	47

This effective sum and therefore the corrosion resistance can be increased by raising the molybdenum content. PRE=1×% Cr+3.3×% Mo (Pitting Resistance Equivalent).

Table 3 shows the results of diverse pitting corrosion investigations. The reduced titanium content has no negative influence on the pitting corrosion temperature. The raised molybdenum content has positive effects.

TABLE 3
Critical pitting corrosion temperature in 6% FeCl3 + 1%
HCl, over 72 hours (ASTM G-48 Method C).
	T in ° C.	Ni	Cr	Mo	N	Ti	PRE
LB 2316	35	39.2	22.4	3.1	0.04	<0.04	33
LB 2317	40	40.1	22.5	4.2	0.03	<0.04	36
LB 2318	50	39.8	22.8	5.2	0.04	<0.04	40
LB 2319	55	38.8	22.9	6.1	0.04	<0.04	43
LB 2320	50	39	22.1	6.2	0.1	<0.03	43
LB 2321	50	40.2	22.6	6.2	0.04	0.4	43
LB 2322	40	40	23.1	6.3	0.1	0.4	44
Alloy 825Reference	30	40	23	3.2	<0.02	0.8	33

Further corrosion investigations likewise revealed an improvement of the critical crevice corrosion temperatures compared with Alloy 825. These are presented in Table 4.

TABLE 4
Critical pitting corrosion temperature (CPT) and
crevice corrosion temperature (CCT)
	CPT	CCT
Alloy	in ° C.	in ° C.	Ni	Cr	Mo	V	Ti	PRE
825*	30	<5						33
PV661	40	15	40	23	3.3	<0.002	0.8	34
PV662	50	20	40	23	5.9	<0.002	<0.002	42
PV663	50	20	39	23	5.8	0.4	<0.002	42

The offset yield strength and the tensile strength can be improved by 15% and 30% cold-working. The associated investigation results of diverse laboratory alloys are listed in the following table.

TABLE 5
Tension tests at RT
				A	Z
Condition	Alloy	Rp0.2	Rm	(%)	(%)
Solution-	825	304	646	—	51
annealed	Reference
	825 Plus (A)	389	754	39	59
		369	772	39.5	61
	825 Plus (B)	390	765	42.5	62
		383	755	40	63
15% cold work	825	670	775	22	71
		697	793	19.5	65
		685	779	23.5	69
	825 Plus (A)	903	973	14.5	51
		893	964	13.5	50
		943	987	13.5	54
	825 Plus (B)	929	974	12.5	56
		877	964	12.5	51
		887	962	9.5	49
30% cold work	825	852	923	14	63
		832	922	13.5	66
		842	920	17.5	64
	825 Plus (A)	979.0	1071.0	11.5	51.0
		970.0	1079.0	8.5	35.0
		996.0	1078.0	11.0	46.0
	825 Plus (B)	980.0	1078.0	11.5	47.0
		980.0	1071.0	11.0	48.0
		996.0	1083.0	10.5	48.0

The following FIGS. 1 and 2 show results of tension tests, on the one hand for the reference alloy 825 and on the other hand for alternative alloys.

Graphical representation of the results of the tension tests at room temperature (mean values) versus condition.

Molybdenum has a positive effect on the offset yield strength and the tensile strength. The positive influence of molybdenum is illustrated in FIGS. 3 and 4.

Graphical representation of the results of the tension tests at room temperature (mean values) versus molybdenum content.

The hot-cracking sensitivity of the Alloy 825, which is an Ni-base alloy, was investigated by means of the PVR test (program-controlled deformation cracking test). The critical crosshead speed V_cr in tension was determined by applying a linearly increasing crosshead speed during TIG welding. The investigation results are illustrated in the following graph. The weldability of the material became better with higher crosshead speed and smaller hot-cracking tendency. The titanium-free, high-molybdenum variants (PV 506 and PV 507) exhibited fewer cracks than the standard alloy (PV 942).

TABLE 6
(chemical composition in wt %)
Heat	C	Mn	Si	P	S	Cr	Ni	Mo	Ti
942	0.006	0.76	0.28	0.012	0.002	22.65	39.42	3.17	0.80
(Prior art)
506	0.01	0.86	0.31	0.005	0.005	23.2	39.0	4.9	0.06
(invention)
507	0.01	0.86	0.31	0.005	0.005	23.2	39.2	5.9	0.06
(invention)
Heat
942	V	Nb	Cu	Fe	Al	Co	B	N	W
(Prior art)			1.94	R30,	0.14	0.11
506				5
(invention)	0.01	0.13	2.4	28.8	0.14	0.28	0.003	0.02	0.10
507
(invention)	0.01	0.13	2.4	28.7	0.14	0.28	0.003	0.02	0.11

The task is also accomplished by a process for the manufacture of an alloy that has a composition according to one of the objective claims, wherein

• • a) the alloy is melted openly in the continuous or ingot casting,
  • b) to eliminate the segregations caused by the increased molybdenum content, a homogenizing annealing of the produced blooms/billets is performed at 1150-1250° C. for 15 to 25 h, wherein
  • c) the homogenising annealing is performed in particular following a first hot forming.

Optionally, the alloy may also be produced by ESR/VAR remelting.

The alloy according to the invention will preferably be used as a structural part in the oil and gas industry.

Product forms suitable for this purpose are sheets, strips, pipes (longitudinally welded and seamless), bars or forgings.

Table 6 compares Alloy 825 (standard) with two alloys according to the invention.

TABLE 6
(chemical composition in wt %)
Heat	C	Mn	Si	P	S	Cr	Ni	Mo	Ti
PV 661	0.006	0.75	0.28	0.003		22.9	39.9	3.32	0.79
(Prior art)
PV 662	0.0066	0.75	0.26	0.003	0.0011	22.9	39.7	5.86	0.002
(invention)
PV 663	0.0071	0.77	0.28	0.004	0.0013	22.7	39.4	5.76	<0.002
(invention)
Heat	V	Nb	Cu	Fe	Al	Co	B	N	Mg
PV 661	<0.002	0.004	1.81	29.8	0.148	0.01	0.003	0.0011	0.012
(Prior art)
PV 662	<0.002	<0.002	1.80	28.4	0.142	0.009	0.003	0.0016	0.01
(invention)
PV 663	0.37	0.004	1.81	28.5	0.155	0.005	0.003	0.0015	0.01
(invention)

Claims

1: Titanium-free alloy with high pitting and crevice corrosion resistance as well as high offset yield strength in the cold-worked condition, with (in wt %)

2: Alloy according to claim 1 with (in wt %)

3: Alloy according to claim 1 with (in wt %)

4: Alloy according to claim 1, which if necessary contains (in wt %) V>0-1.0%, especially 0.2-0.7%.

5: Process for the manufacture of an alloy that has a composition according to claim 1, wherein a) the alloy is melted openly in continuous or ingot casting,b) to eliminate the segregations caused by the increased molybdenum content, a homogenizing annealing of the produced blooms/billets is performed at 1150-1250° C. for 15 to 25 h, whereinc) the homogenizing annealing is performed in particular following a first hot forming.

6: Use of the alloy according to claim 1 as a structural part in the oil and gas industry.

7: Use according to claim 6, wherein the structural parts exist in the production forms sheet, strip, pipe (longitudinally welded and seamless), bar or as forging.