While both cast and forged production can deliver high quality valves, it isn’t always obvious which has the best cost-performance ratio for applications in the power industry, writes Arvo Eilau
The cast versus forged debate has been around for years in the valve manufacturing industry.
Although cast production still dominates, changing operating environments in the power industry, such as the shift towards higher pressure and temperature applications, have reignited the debate on which technique is most suitable to manufacture valves which meet these evolving application requirements.
While there is no doubt that both cast and forged production are capable of delivering high quality valves, it isn’t always immediately obvious which solution has the best cost-performance ratio for different applications in the power industry.
Driven by the ever-increasing demand for energy, operational temperatures in supercritical and ultrasupercritical power plants as well as in combined-cycle power plants have climbed significantly in the last two decades. Testing in Europe is currently at 700°C/1292°F for boiler systems and plants in the US will soon be operating above 600°C/1112°F. While multiple thermal cycles and higher temperatures allow operators to increase plant efficiency, they also put an immense amount of stress on system components. Degradation of material caused by creep and corrosion fatigue are now being detected more often in modern CCPPs and heat recovery steam generators (HRSGs), leading to higher maintenance levels and plant downtime. As a result, many valve manufacturers have started to rethink their material specifications and production techniques, in particular when dealing with components for critical and severe service applications.
Changes in the operating parameters of modern power plants, and in particular the move from static to dynamic loads through combined-cycle operations, affect valve material behavior.
Increases in plant cycling impact on temperature and pressure differentials, affecting the load placed on valves and pipeline components within the steam circuit.
Fluctuations in temperature create stress and elongation as the valve expands and contracts under different loads up to a designed ‘yield strength’.
Deformation of the valve is caused once stresses placed on the valve exceed this yield strength by affecting the dimensions of components over time, with mechanical stress eventually causing component fatigue and failure.
In general, all valve materials, whether forged or cast, have to fulfil the demands for tensile strength and corrosion resistance that can be found in many critical process envrionments.
The stronger the valve, the better it is able to resist failure when put under pressure, resulting in higher ductility and ultimately longer product lifetime. However, a key application feature in the power generation industry is the potential for valve failure due to the increase in plant cycling to meet energy supply needs.
The impact of high cycling duty on steam temperature and pressure places greater stresses on valve components, and requires the material to endure these fluctuations and differentials throughout the lifetime of the valve.
Cast materials tend to require less mass than forged products – more mass can lead to larger thermal gradients which can result in greater stresses. The relationship of casting and potential voids needs to be taken into account, with consideration given to determing the safety factor and allowable stress level.
Another important aspect that needs to be considered is the metal’s ability to withstand the pressures of corrosion. Caused by the combined actions of oxygen, other metals and salt, corrosion is still a major problem in the power industry, often resulting in costly repairs and excessive downtime. Degradation of valve material is further accelerated when under pressure from high operating temperatures. The higher the temperature, the softer the material will become, increasing the risk of thermal fatigue.
Design codes and standards from the American Society of Mechanical Engineers (ASME) allow for the use of both forged and cast materials within the primary circuit, depending on the boiler design and operational parameters. The chemical composition of these components, in particular the level of chrome, and the manufacturing method are key factors to determine a valve’s creep strength and metallurgical stability. For example, WC9 grade alloys, containing 2.25 per cent of chrome, can retain their strength in temperatures of up to 550°C, while higher grades, such as C12-A with 9 per cent chrome, are able to withstand temperatures of up to 610°C.
To cope with the changes in power plant operations, the situation for forging grades is beginning to evolve. While lower grades, such as F22, containing 2.25 per cent of chrome, can be suitable for up to 590°C, F91 materials with 9 per cent chrome are able to withstand temperatures of up to 650°C. Increased efficiency targets for modern power plants are pushing operating temperatures above 600°C. To meet these requirements, plant engineers and valve manufacturers are moving to F92 grade alloys, for which there is no cast equivalent within the ASME standard. The difference in performance over F91 grade alloys is also clear. For example, an F92 class 2500 valve at 600°C delivers a special class rating of 223.4 bar compared with 203.1 bar for F91. At 575°C, the ratings are the same between the two materials. However, at 625°C, the difference is 190.6 bar for F92 compared with 152.1 bar for F91.
Although chemical composition is an important consideration as it has an impact on the mechanical properties of the material, the heat treatment of the material is an essential parameter that determines the overall strength of the valve when operated in high-temperature environments and under intense mechanical stress. Whether forging or casting, each valve will need to be heated to a normalized temperature – usually around 1000°C – and needs to be cooled down in less than 4000 seconds to achieve a highly durable end product. While smaller valves can be cooled down relatively quickly, valves with higher mass, typically with a large diameter, have a much more complex cooling-down process. This in turn will affect the mechanical properties of the valve, and in particular its performance in high temperatures, which could lead to a reduced product lifecycle. Heat treating materials is a crucial step that requires each manufacturer to maintain specific procedures.
Meeting quality standards
While the majority of international standards provide guidance on the expected guaranteed life of the material in ambient temperature, there is currently no official data available for tests in high temperatures. However, the changing operating environments in the power industry, in particular the stresses caused by higher cycling and regular shutdowns, are calling for changes in testing procedures. As a result, tests that take into consideration additional parameters, such as operating hours, number of shutdowns and temperature range, are becoming increasingly important in order to accurately evaluate the lifespan of the product.
With changes to material guidelines and regulations imminent, many engineers have already started to specify materials that go beyond the current required specification in order to secure future compliance. In fact, some governing bodies, such as the American Society of Testing and Materials (ASTM) and ASME, have already started to include detailed manufacturing process requirements into their specifications in response to the changes in the operating environment. Besides fulfilling stringent criteria in terms of the chemical composition, testing procedures and physical properties of the material, many valves now also have to be either cast or forged, depending on the industry and application. Ultra-supercritical coal-fired plants, for example, are capable of operating above 600°C, making these applications ideally suited to F92 forged grade materials. In the coming years, the combined-cycle market will move towards these temperature ranges and the use of F92 is expected to become more prevalent in these applications.
Suitability of valve casting
Casting still represents a large share of the valve manufacturing industry. In fact, most steel components start as castings, a process in which molten metal is poured into a customized mould and then solidified. There are a number of advantages to this procedure:
• Flexibility in design as valves can be cast-to-shape (greater variety and complexity of shape as processed in liquid form);
• Greater metal choice (custom alloys) as foundries have full control over the chemical metal composition to meet unique requirements at an affordable cost. Can ensure valves are modified to meet exact specifications (i.e., can control the level of ferrite to enhance corrosion resistance, etc.);
Credit: Pentair Valves & Controls
Micro graph of a typical F91
Credit: Pentair Valves & Controls
• Reduced machining costs (require less machining than forgings to achieve more complex shapes);
• Castings are more widely available, making replacements easier;
• Cast valves have a contoured shape (rounded edges).
Although castings remain an integral part of the valve manufacturing industry, the shift towards higher pressure operation in the last few years has exposed a number of shortcomings:
• The solidification process can produce small impurities, such as voids and cracks that can lead to lower mechanical properties, requiring costly and time-intensive weld repair;
• Welding and post-weld heat treatment alters the microstructure of castings, resulting in higher creep and much lower hardness. This requires rigourous inspections of the valve to ensure it has maintained the proper strength and creep resistance;
• Higher specifications in 9-chrome material can only be achieved after extensive heat treatment, resulting in longer delivery times which can be caused by many factors including NDE and testing requirements.
However, there have been significant improvements in the mould- and core-making processes, reducing the appearance of defects on casted valves. For example, low pressure die casting techniques are adopted by an increasing number of foundries to improve the overall mechanical integrity of the final product.
The rise of forged valves
The past few years have seen a shift towards the use of forged steel, in particular for valves used in critical and severe service applications. This is reflected in the use of higher performance alloys in temperature applications above 600°C/1112°F. All forging processes start with a solid piece of metal or ingot that is forged into shape with hammers or presses. Although work-intense, this process has a number of benefits:
• Less material waste as less re-working is necessary (valve is forged into shape out of one solid piece/ingot);
• Forging process reduces surface porosity and closes up internal cavities and voids due to the immense pressures involved in the manufacturing process. This enables forged valves to retain their structural integrity, resulting in a mechanically stronger and more durable product (higher ductility and tensile strength);
• Flexibility of the forged material being manufactured to the intermediate rating. This has less wall thickness for enhanced performance due to thermal cycle fatigue with 9Cr-1Mo material – less wall thickness has a smaller temperature gradient which requires less time for material thickness to reach equilibrium, and is therefore less prone to thermal fatigue. This offers a robust solution for power plants which cycle through startup and phase-down on a daily basis;
• Forged material can be machined to meet specific design conditions using intermediate ratings. This offers end users and asset owners another alternative and can be used in both valve castings and forgings. With the use of an intermediate rating, a lighter weight valve can be produced that can be more suitable to cycle duties. The lighter weight valve is able to reduce the heating/cooling stresses as the units cycle on and off, particularly as thinner wall components can heat and cool more quickly, resulting in fewer thermal stresses.
|Forging stainless steel|
While forging produces a very strong piece of equipment, there are some consequences and limitations to be aware of:
• Cost- and energy-intensive (requires extensive work in order to refine the product and achieve the required shape and finish);
• Limitations on size, shape and thickness (processed in solid state);
• Larger forgings need to be produced from two or more pieces and welded together.
Right valve, right application
High manufacturing standards and attention to detail are paramount to ensure that each valve, whether cast or forged, meets the required design and performance criteria. While forged materials will certainly start to dominate high-pressure and high-temperature applications in some industries, casting will continue to provide a cost-effective and reliable alternative and remain an important asset to the valve manufacturing industry.
Ultra-supercritical plants are ideally suited to F92 forged materials
Ultimately, evaluating the operational needs of a specific plant will identify the best valve solution in terms of forged or cast components. While forged valves offer enhanced performance for high- temperature and severe service applications, there are restrictions in terms of cost and repeatability for standardized products. The key consideration for plant engineers assessing cast components is identifying the long-term effect of an acceptable void within the valve’s construction once it is installed. Will plant operators accept a small casting void in a valve that will cycle many times a day? A solution to mitigate these risks is to work with an experienced valve manufacturer which offers engineering expertise in both cast and forged valves, and can advise on the most appropriate ASME-compliant product which meets specific plant and application needs.
Arvo Eilau is Marketing Manager for Gas Power at Pentair Valves & Controls