GB2230024A

GB2230024A - An heat treatment for seamless steel pipes

Info

Publication number: GB2230024A
Application number: GB9004336A
Authority: GB
Inventors: Hagen Ingo Von; Fritz Kegel; Heinz Sanders
Original assignee: Mannesmann AG
Current assignee: Vodafone GmbH
Priority date: 1989-02-28
Filing date: 1990-02-27
Publication date: 1990-10-10
Anticipated expiration: 2010-02-27
Also published as: GB2230024B; FR2643648A1; JPH02277720A; GB9004336D0; DE3906700C1; FR2643648B1

Description

1

Process for Rroducing seamless oilfield pipes

The invention relates to a process for producing seamless pipes. It is particularly concerned with the production of pipes for oilfield use of grade 80 or 95 ksi. Pipes of the invention are produced from an Alkilled chromium steel of the following composition:

0.17 - 0.23 c 0.20 - 0.50 si 0.40 - 1.00 Mn max. 0.02 p max. 0.01 S 0.010 - 0.035 Al 12.0 - 14.0 Cr max. 0.50 Ni max. 0.10 v 0.020 0.060 % N the balance being iron and unavoidable impurities.

It is known to roll out case-hardening blocks of the above steel composition into seamless pipes in several stages after heating to a case-hardening temperature in the range of 1150 - 12500C. After the last shaping step, the pipes are cooled to room temperature in air. Then they are optionally subjected to stress-free annealing in order to avoid stress cracking corrosion. In order to set the required mechanical properties for oilfield use, the pipes are then subjected to a separate hardening and tempering process, corresponding to the API standard valid for such pipes, which comprises air-hardening with subsequent tempering. In this case, provision is made for heating to at least 9500C for air-hardening. The tempering temperature depends on the grade, and is 1 2 usually 715C for-grade 8.0 ksi and 6900C for grade 95 ksi, with the- coalIng again taking place in air. The mechanical properties.produced after the heat treatment must satisfy the following requirements:

d' Grade ksi ksi Elastic limit Rp05t 552 - 655 N/nM2 655 - 758 N/MM2 Tensile strength Rm k 655 N/inm2 >- 724 N/IRM2 Elongation at break A21 17 % k 15% (according to API formula) Hardness < 23 HRC 5 26 HRC _/1 at Wd 5 12. 7 mm 3 HRC 3 HRC -,at Wd > 12.7 mm 4 HRC 4 HRC up to 25.4 mm Notched bar impact work Av-10C minimum individual value 35 J not required mean value k 45 J W 4 The heat treatment carried out after the pipes have been rolled uses considerable quantities of energy and of course requires suitable apparatus for the heating and cooling operations. This has a considerable influence on the production costs.

The present invention is directed at a generic process for producing seamless oilfield pipes from the above-mentioned chromium steel with lower energy consumption and hence greater economy, without substantial loss of the mechanical properties of the pipes. In particular, the invention is directed at an heat treatment for hot-rolled pipes of the above mentioned steel composition.

According to the invention, an heat treatment is applied to seamless steel pipes, normally of grade 80 or 95 ksi, produced by multi-stage hotrolling of case-hardening blocks which have been heated to a casehardening temperature in the range of 1150 - 12500C. The treatment comprises cooling from a temperature in the range of 960 - 9900C to room temperature at a cooling rate of 0.5 - 5 K/s; tempering without a transformation by separate hardening, at a temperature which is 10 - 30 K above the conventional tempering temperature for the relevant grade of steel required. The tempering temperatures in treatments according to the invention are in the range of 725 - 7450C for grade 80 ksi and in the range of 700 to 7200C for grade 95 ksi, depending on the chemical composition. The treatment is conveniently carried out as part of the production process, with the cooling step being directly from the rolling heat after leaving the last hot rolling stage.

In the production of steel pipes, it is indeed known that after the pipes have been quenched from the 1 rolling heat it is possible to reach a hardened and tempered state by subsequent separate tempering. However, this is problematical due to the deterioration of the toughness properties which is often noted as a rule compared with conventional hardening and tempering treatment, and therefore cannot be permitted in many cases. Separate hardening leads to a structure which is more favourable for the toughness properties due to the refining of the grain involved with the transformation. It was therefore completely surprising that in the case of the steel pipes produced according to the invention, despite the lack of transformation upon quenching from the rolling heat compared with conventional hardening and tempering, as well as the desired strength properties not only the same but even considerably improved toughness properties could be achieved.

The invention will be explained in greater detail below with reference to two embodiments. In both cases, a steel having the same chemical composition was used:

0.18 c 0.26 si 0.49 Mn 0.016 p 0.003 S 0.022 Al 12.73 % Cr 0.10 % Ni 0.08 % v 0.05 % N the balance being iron and unavoidable impurities The case-hardening blocks were drawn at 1220C and rolled out to seamless pipes having a diameter of 88.9 mm and a wall thickness of 7.24 mm in known manner.

1 1 1 t 6 After finish-rolling, the pipes each had a temperature distribution in the range of 960 - 9800C over the pipe length and were therefore in the narrow temperature window of 960 - 9900C to be maintained according to the invention as the starting point for cooling. The cooling was carried out in controlled manner on a cooling bed with a mean cooling rate of 1.5 K/s.

In order to set grade 80 ksi, a first part of the pipes was tempered for 60 min at 730C and then cooled in air. Then the following mechanical properties were noted on these pipes:

RpO.5t 555 - 648 N/MM2 (mean value 623 N/M1112) RM A21 793 - 832 N/MM2 (mean value 813 N/MD12) 17 - 25 % (mean value 21%) Hardness = 20 - 23 HRC 87 - 205 J (mean value 155 J) (In the examination, 2/3 samples were used, which were re-calculated to complete samples with factor 0.67) A second tube part which had been tempered for 60 min at 7200C in order to set grade 95 ksi had the following mechanical properties after cooling in air:

Rpo. 5t 659 - 698 N/MM2 (mean value 682 N/MM2) RM 818 - 851 N/M1a2 (mean value 838 N/IM2) A2f 15 - 23 % (mean value 19%) 7 Hardness = 23 - 26 HRC Since no notched bar impact work values are yet required for grade 95 ksi, these were not measured. The value found for the elongation at break of 19% on average is considerably above the required minimum value of 15%, and gives rise to very good toughness properties.

Seamless pipe production processes according to the invention lead not only to a considerable reduction in production costs as a result of dispensing with a separate hardening stage, but also achieve improvements in the mechanical properties of the pipes produced.

1 8

Claims

1. An heat treatment for seamless steel pipes produced by multi-stage hotrolling of case-hardening blocks which have been heated to a casehardening temperature in the range of 1150 - 1250C and which have the following composition:

0.17 0.20 - 0.40 max.

max. 0.010 12.0 max.

0.23 0.50 1.00 0.02 0.01 0.035 14.0 Cr 0.50 Ni max. 0.10 v 0.020 0.060 N the balance being iron and unavoidable impurities, which heat treatment comprises after leaving the lasthot rolling stage, cooling from the rolling heat of a temperature in the range of 960 - 9900C to room temperature at a cooling rate of 0.5 - 5 K/s; tempering without a transformation by separate hardening, at a temperature 10 - 30 K above the conventional tempering temperature for the relevant grade of steel required.

2. A treatment according to Claim 1 wherein the tempering temperature is in the range 725 - 745C for a seamless pipe of grade 80 ksi.

3. A treatment according to Claim 1 wherein the tempering temperature is in the range 700 - 720C for a seamless pipe of grade 95 ksi.

c si mn p S Al

4 i l 9 4. A treatment according to any preceding claim wherein the composition of the steel is:

0.18 c 0.26 si 0.49 Mn 0.016 % p 0.003 % S 0.022 % A1 12.73 % Cr 0.10 Ni 0.08 v 0.05 N the balance being iron and unavoidable impurities.

5. An heat treatment for seamless steel pipes according to Claim 1 and substantially as herein described.

6. A process for producing seamless steel pipes comprising the multistage hot-rolling of case hardened blocks heated to a case-hardening temperature of 1150 to 12500C, including heat treatment according to any preceding Claim.

Published 1990 at The Patent Mce, State House.6671 High Holborn. London WC1R 4TP. Purther copies maybe obtained&om ThePatantOffice ace& Branch, St Mary Cray. Orpington, Kent BR5 3RD. P=ted by Multiplex tach=ques it& St Mary Oray, Kent. Con. 1187