Advancing a metallurgical journey into the future of bar and hollow bar

It all started with a question

Bertil Waldén is no stranger to asking engaging questions. As Principal R&D Professional and Senior Project Leader at Alleima (formerly Sandvik), he’s always looking for smarter ways to advance industry through sustainable innovation and materials that exceed vendor requirements. But what surprised him about the Sanicro^® 925 (UNS N09925) bar qualification was the warm response his team received from the oil and gas industry all over the world.

“The industry response to our question about how we could jointly advance their success has been overwhelmingly positive,” says Waldén. “During the one-year qualification process, we often had Teams meetings with up to 20 metallurgists on two continents from six of the world’s largest oil and gas companies, as was well as oilfield companies and our own guys in the oil and gas hubs.”

“It felt like a giant brainstorming session, with everyone coming up with suggestions for C-ring testing, slow strain rate testing (SSRT), and even suitable independent test labs.”

But why so much interest in a nickel alloy that’s already on the market? According to Waldén, there are several reasons why OEMs are all working side-by-side with Alleima to support the qualification:

Key challenges

• Global shortages of Alloy 925 bar / hollow bar stock
• Difficult to manufacture; long lead times
• Recent issues with hydrogen em­brittlement
• Upgraded API 6A CRA Standard
• Extended oil & gas requirements

Resisting hydrogen embrittlement

He points to some recent material failures relating to the installation of downhole OCTG casings and tube strings in sour high-pressure wells with CO₂ and H₂S. When it comes to drill string components (a key application), precipitation-hardened steels like Alloy 925 have proven sensitive to hydrogen induced stress cracking (HISC) and cathodic corrosion. Due to unacceptable microstructures, the oil and gas industry has also experienced some failures with Alloys 718, 725 and 716.

New microstructure demands

In 2019, such concerns led to the esta­blishment of a Joint Industry Project (JIP), as reported in NACE Paper 13284, which included specific recommendations for tougher standards for slow-strain-rate testing (SSRT). It also led to the extension of certain API tests, including grain boundary for microstructures and more. Alleima is now the first and only manufacturer to carry out this qualification – not only the latest API 6A CRA 5P, but even more stringent customer requirements.

Longtime partner to O&G sector

Meeting high safety and performance standards for the oil and gas sector is nothing new to Alleima. For more than 60 years, the company (formerly Sandvik) has been a trusted partner to all major players. Today, larger volume OCTG products include 22Cr, 25Cr, Alloy 28, Alloy 825, and G-3, i.e., group 2-4 materials. Plain end tubing is provided via Tenaris with their threaded coupling. In addition, Alleima offers world leading SAF™ 2507 super-duplex hydraulic and chemical injection lines for subsea umbilical tubing and many other applications requiring strong corrosion resistance.

If you want to play on this level, you need to listen and deliver

Sanicro^® 925 dimensions and formats

Alloy 925 – Sanicro^® 925

• Rolled Bar 3" – 4.1/2"
• Forged Bar 4.1/2" – 9" (Trials aim for 12")
• Hollow Bar 3" – 10" OD

An ideal OCTG complement

“Sanicro^® 925 is an ideal complement to our OCTG portfolio and growing Sanicro family,” says Marcus Hillbom, Technical Marketing Manager at Alleima. He notes that the rollout of Sanicro products – the company trademarked name for nickel-based alloys – is part of a larger strategy to add value in new niches, reduce stock shortages and extend customer possibilities.

Growing Sanicro range

In 2021, Alleima (then Sandvik) launched an entry level Sanicro^® 825 (UNS N08825) in bar and hollow bar; and in 2022, Sanicro^® 625 (UNS N06625) bar. Next out will be Sanicro^® 718 (UNS N07718) which, like Sanicro^® 925, will undergo an extensive validation process.

925 and 718 in a different league

“Our new Sanicro^® 925 – and soon 718 – bar and hollow bar products are in a completely different performance league than 825 and 625 products,” says Hillbom. “Sanicro^® 925 is an age-hardened version of Sanicro^® 825 that offers roughly four times the strength.

And, when qualified, Sanicro^® 718 will provide significantly higher strength and hardness than all other Sanicro grades. In other words, these grades are a bit special, and to become an approved vendor, we’re having to collaborate closely with some of the toughest customers in the business.”

Meeting special requirements

Waldén is in complete agreement with Hillbom when it comes to the special requirements of the OEMs and Tier 1 players. “If you want to play on this level, you need to listen well and deliver,” he says. After thousands of hours of customer input, he and his team identified some serious concerns about corrosion in well completion drill string and other components.

A common theme was hydrogen embrittlement challenges relating to cathodic protection, electric circuits and bar components not matching the strength or microstructure of other OCTG materials. Also, some suppliers were not meeting the latest, more stringent test recommendation.

Key objectives

Based on this input, Bertil and his team laid down a program with the following objectives:

• Strengthen collaboration with customers
• Advance customers’ profitable and sustainable business
• Gain valuable insights into key challenges
• Meet all requirements for API 6A CRA plus new, extended tests
• Standardize quality in Ni bar/hollow bar manufacturing
• Build valuable knowledge we can share with industry

Scope of 925 qualification testing

“In addition to standard tests for mechanical strength, microstructure and corrosion, we also received requests for independent quality verification of new specifications,” says Bertil Waldén. “Some customers wanted these tests done in Houston, others in France, so we did them in both locations. Before sending any samples, all heats were witnessed by a third party: TÜV NORD.”

Inclusive, multidisciplinary approach

The project was initiated and led by the R&D department of the Tube Division with the aim of developing the technology needed to add Alloy A925 and Alloy A718 to the Alleima offering. To ensure the best outcome, Bertil Waldén involved members from different departments within Tube R&D as well as central R&D competencies – physical metallurgy, electron microscopy (TEM/SEM), mechanical testing, corrosion science, heat treatment, process metallurgy and hot working.

The focus of the tests was the following

Three heats of Sanicro^® 925 solid bar in the diameter of 185 mm (7 ¼ inches)

One Sanicro^® 925 hollow bar batch with 165 mm OD x 25.4 mm wall (6 ½ inch OD x 1 inch wall)

Scope of project

• Update Quality System for bar manufacturing
• Meet all criteria for API 6A CRA standard:
• Chemical composition
• Mechanical testing
• Tensile strength
• Charpy testing
• HRC testing
• Grain size (microstructure)

Sold bar Non-destructive testing to API 6A PSL 4

90-day C-ring testing in accordance with NACE TM0177, Class VII Environment

Hydrogen embrittlement SSRT Testing to NACE Paper 13284

Technical results available upon request

In this article, we do not provide specific data, although our technical results met or exceeded industry requirements, as confirmed by independent testing laboratories in Europe and the USA. Our aim here is to review key approval steps by the American Petroleum Institute (API 6A CRA) for safer valves and wellhead equipment in sour and aggressive conditions, as well as new independent tests being recommended. Please contact us to see the data, organize an audit or discuss further.

Highlights of testing and improvements made

Independent testing took us to Houston, France, Sweden and back again

From melt to final bar

Uniquely, the central R&D department of Alleima is located just a few hundred meters away from the company’s own steelworks (melt shop, rolling & forging, tube production lines, etc.) in Sandviken, Sweden. As a result, the R&D team can easily communicate and collaborate closely with the company’s “Primary” (PU) production unit to optimize results.

Full traceability

As Bertil Waldén put it: “We are lucky to have a steel mill in our backyard. It means we can control and monitor the chemical and mechanical properties at every step, with full traceability.” In Sandviken, he says, Alleima uses an electric arc furnace, AOD converter and then (after ingot casting and vacuum arc remelting) has its own blooming mill, rolling mill and forging press to achieve various formats, as needed (see Figure 1). In particular, he says that “forging parameters are the crucial step to achieving the correct microstructure.”

Sustainable bar profile

All steel is made with 83% recycled steel and produced using fossil-free energy, in accordance with ISO 9001 certifications. This is in line with Alleima’s ambition to lead on sustainability and circular thinking. The company adheres to the UN Sustainability Goals and has signed the Science Based Targets initiative (SBTi) to reduce carbon emissions by 50% by 2030.

Materials testing and chemical analysis, as well as the department of calibration of furnace control and thermocouples are organized alongside each production unit.

Improving the Quality System

The Alleima Quality System is already certified in accordance with ISO 9001, with each division taking individual responsibility for its own Quality Assurance and Continuous Improvement system. When it comes to the manufacturing of precipitation hardened (PH) nickel-based alloys (e.g. Alloy A925 bar and hollow bar), we have carefully laid down specific instructions for:

• Seamless hollow bar in Alloy 925
• Turned bar in Alloy 925
• Extrusion bar in Alloy 925

New quality procedures have now been implemented, based on the tougher requirements for these
superalloys on API 6ACRA in all phases – from grinding and heating to peel turning and straightening.

Material and Chemical Composition

Sanicro^® 925 is our first-ever precipitation hardened nickel-iron-chromium alloy in bar and extruded hollow bar, with additions of molybdenum, copper, titanium, and aluminum. It is designed to offer a combination of exceptional mechanical strength, resistance to environments containing hydrogen sulfide and stress corrosion cracking (SCC) in chloride bearing environments.

Tougher testing of 4 x 925 heats

To determine the optimal chemical composition for different processing conditions (grinding, peeling, etc.), we produced five AOD heats with different lot numbers and a combined weight of 300 metric tons — all to the requirements of API 6ACRA. The forging ratio was 6:1. During a 12-month period, we then upgraded our testing to meet the even tighter requirements of ASM 2750. Solution annealing was carried out
to Class 5 specs and precipitation hardening to Class 3 requirements.

Mechanical Testing and Microstructure

A number of tests of mechanical properties at room temperature (and elevated temps) of solution-annealed and solution-annealed plus aged products were carried out according to API6 CRA:

• Tensile strength test: The material test data for 3 heats of bar and 1 hollow bar showed material strength and yield strengths that exceeded MPa/ksi (160-180) requirements of API6 CRA.
• Charpy V-notch test: This impact test was performed on samples down to -60° C/-75 °F in accordance with ASTM E23. All specimens passed the test with a good margin.
• HRC Testing: Hardness testing results were in the high end of the standard, according to API6 CRA, and in accordance with Rockwell scale C, as specified in the EN ISO 15156-3:2019 standard.
• Grain size / Microstructure: Microsamples were taken from the surface, half radius, and center locations of the bar. Macro etching was done according to ASTM A604 Class 1-4, less than severity 4. No topological duplex structure was seen in the samples per ASTM E122. Grain boundary precipitates were equivalent to or better than the API 6A CRA standard and thus fully compliant.

90-day C-ring testing in accordance with NACE TM0177, Level VII environment

Based on the recommendations of metallurgy engineers at its customers, Alleima engaged Arista Laboratories in Houston, Texas to perform advanced, independent autoclave C-ring testing. This test was done on three heats of Alloy 625 to evaluate the material’s resistance to stress corrosion cracking (SCC) in NACE Level VII environments – that is, environments consisting of 25 wt. % NaCl solution saturated with 500 psi H₂S and 500 psi CO₂ at a test temperature of 401 °F (205 °C).

• No fracturing, no cracking
  The outcome of these tests showed that all C-ring SCC specimens of Alloy 925 completed the 90-day exposure without fracturing. Additionally, testers at Arista observed no evidence of cracking during post-test visual examination at low magnification.
• No sub-surface cracking
  In a second step, all C-ring SCC specimens of Alloy 25 were sectioned circumferentially and metallographically examined at up to 500X magnification. No cracking or sub-surface cracking was observed in any of the specimens (see example above).

SSRT Testing in accordance with NACE Paper 13284: 2019

Yet another request for independent testing relates to drill string challenges with hydrogen-induced stress corrosion cracking (HISC). In recent years, this had been a challenge for O&G OEMs and Tier 1 suppliers. In 2019, the topic was addressed in NACE Paper 13284, a joint industry initiative that investigated 28 heats of PH Ni alloys, or eight materials grades, from different manufacturers.

The study evaluates “acceptable” and “unacceptable” microstructures (mainly for Alloy 718) and makes joint industry recommendations for tighter testing on precipitation-hardened bar for HISC resistance in the microstructure – that is, in addition to API 6ACRA.

SSRT testing in France

Says Bertil Waldén: “Our customers asked us to follow these recommendations based on the earlier NACE 13284 study, which is essentially an extension of the API 6A CRA standard. No other manufacturers have done these extended tests for Alloy 925 yet. They also suggested we use Institute de la Corrosion in Brest, France for Alloy 925 (718 to follow) since they had experience with the test method in the Joint Industry Project (1,2,3)”

Taking a mix of relevant samples

SSRT testing involved three heats of forged bar and one heat of hollow bar with the locations of the samples shown above. One set of tests was taken near the maximum thickness for each production route (each 120 degrees from each other, at half radii and half wall, as shown above). All testing was carried out in accordance with NACE Paper 13284: 2019.

Environmental conditions used

Environmental test conditions were 0.5M sulphuric acid, with an Ar-10%H2 mixture continuously bled into the test chamber, under applied cathodic production of 5mA/cm2, with test temperature of 40°C. The strain rate was 1.0x10-6 s-1. Evaluation of specimens was done as per NACE TM-01-98. Test acceptance criteria for elongation ratio as per 0.45 min (Ave), NACE Paper 13455.

Back to Houston for more SSR testing

After the samples passed the French tests, one of the major operators advised us that Arista Laboratories in Houston was the only laboratory qualified by them for HE / SSR Testing. As a result, we repeated the same SSR tests on the three Alloy 925 bars again in the USA. This also has the advantage of being able to compare tests results from more than one laboratory. In both instances, the bar samples passed the tests with superior results, well within the minimum.

What we learned and how can it advance your success?

Now that Sanicro^® 925 has met all key acceptance criteria, five out of six of the world’s leading oil and gas companies have approved Alleima as a qualified vendor. The company is currently booking orders to the oil and gas industry for 2023 and in the years ahead.

Roadmap to the future

“We now have all the relevant certificates and welcome any customers or partners who require a site audit or would like to discuss in further detail, says Bertil Waldén, who further reflects on the process: “The qualification of Sanicro^® 925 required a lot of listening, significant investments, and a commitment by all parties. It provides a roadmap for developing a future generation of materials. We’ve learned a lot and can share vital new information with our trusted customers and partners.”

Key achievements and outcomes

All new Sanicro^® 925 grades fully meet API 6A CRA

Four different AOD heats of Sanicro^® 925 (UNS N09925) have now been produced and qualification tested (three in bar and one in seamless hollow bar). All material heats meet the chemistry, mechanical properties, and metallographic requirements of API 6A CRA.

Three bars pass C-ring testing to NACE TM0177, Level VII

The three bars have been subjected to C-ring testing and independently verified at the Arista Labs in Houston, Texas. All bars passed the NACE TM0177 (Level VII) tests without any remarks.

Three bars met SSRT testing to JIP in NACE paper 13284 in EU and US

Slow strain resistance (SSRT) testing was carried out on the same 3 bars from 3 different AOD heats and one hollow bar from a fourth heat at independent test centers in the France and the USA. In all cases, the products passed the tests, with an average elongation ratio varying from 0.57, 0.66, 0.93 for the bars, confirming the suitability of the Alleima metallugical process.

One hollow bar met SSRT tests (NACE 13284) in EU and USA

For the hollow bar, the average elongation ratio in SSRT testing was 0.73. In other words, we saw similar resistance against hydrogen embrittlement in all the different product forms.

New higher product routines standardized and in place

Alleima improved its quality and standardized steps to secure the very highest production results, including administrative processes to for safety, consistency and to ensure full compliance with the highest-level international standards for CRAs. This includes new sustainability standards.

Your additional source of premium bar & hollow bar stock

All of the above mean that industry gains an additional trusted, high-quality source of Sanicro^® 925 grades that meet the API 6A CRA standard as well as additional stringent requirements. The products are made available through our global operations and trusted distributors. For further information about testing data, to schedule an on-site audit or discuss your challenges, feel free to get in touch.

References

1. Trillo et al, NACE Corrosion 2019, Paper 13455
2. Duret-Thual et al, NACE Corrosion 2019, Paper 13365
3. Salvatori et al, NACE Corrosion 2019, Paper 13365
4. NACE TM 0177