ETD Ltd, Ashstead, Surrey, UK
ABSTRACT
A variety of new austenitic and nickel based alloys have been developed in the 1980s and 90s, many of which are finding application in advanced energy conversion plant.
The paper briefly examines the historical background to these new materials. They can be traced back to the simple austenitic stainless steels of the 1930s, in both their wrought and spun cast forms, and in the development, in the late 1940s, of super stainless alloys such as Incoloy 800.
Most of these alloys have been developed outside of the boiler plant materials programmes aimed at conventional power generation systems. These programmes have largely focused on low alloy ferritic, bainitic, and martensitic superheater tube materials, intended for use at fairly modest temperatures and in moderate corrosive conditions. The advanced alloys, covered in the paper, are used in much more aggressive conditions, in terms of temperatures and in the severity of corrosion.
Both are characteristic of novel energy conversion processes requiring advanced forms of heat exchanger. Examples of alloys from Europe, America, and Japan are used to describe some of the thinking behind the development of these new mate- rials.
The background to commercial and near commercial energy conversion systems, which require advanced heat exchangers, is also described. These include indirect fired gas turbines; recuperative gas turbines of various types, fluidised bed com- bustion, coal gasification, and waste incineration.
To optimise strength over a given temperature range, each of the new alloys needs to utilise a specific type of precipitation or dispersion strengthening mechanism. In the older austenitic alloys the principal precipitation processes involved gamma prime and chromium carbides. The newer alloys utilise yttria for dispersion strengthening, and nitrides, carbo-nitrides, complex carbides, and copper rich phases for precipitation hardening.
It is emphasised that the general fall off in stress rupture properties, with tem- perature, is governed by the ease of dislocation climb over strengthening precipitates.
It is therefore possible to draw an 'average' stress rupture strength line for austenitic and nickel based alloys, the gradient of which is probably governed by the diffusion rate and elastic modulus of such materials.
In order to maximise resistance to attack in advanced heat exchanger environ- ments, the newer materials need higher levels of alloying elements. In general, chromium contents are very much greater, particularly where the attack involves a molten salt phase. However, at very high temperatures, to resist simple oxidation, the newer alloys need to contain significant amounts of aluminium, so that in that in some cases they form scales of alumina rather than chromia. This feature has been
made possible by the addition of active elements, so reducing the propensity to scale spallation. Recent progress with the understanding of coal ash corrosion is de- scribed.
Two major problems remain to be overcome in a satisfactory manner. These are corrosion in coal gasification and waste incineration systems. In both processes there is now a reasonably good understanding of the complex mechanisms that lead to such high rates of corrosion at such comparatively low temperatures. The presence of high levels of chlorine creates new difficulties. This points to the need for devel- opment of purpose designed alloy systems and coatings. Spun cast production of high alloy tubing may give new opportunities in this respect.
1 INTRODUCTION
The past decade has shown a refreshing change in the development and use of materials for heat exchangers. In some cases materials have been specifically developed to resist new and harsher conditions. In others, experience with new alloys has been shared between diverse industries and technologies. A principal aim of this paper is to identify these common features and to indicate how progress may be further accelerated.
Rather than to produce a compendium of commercial, near commercial, and experimental alloys and coatings, a more selective approach will be adopted in this paper, focusing on the alloy design aspects of high temperature heat ex- changer materials. The chief needs for such a material, if it is to be used to construct a heat exchanger, is that it must have adequate creep strength at temperature, must have reasonable resistance to high temperature corrosion, and must, normally, be capable of being fabricated in a tube form. These require- ments are often in conflict. It is one reason why, with the need for exchangers to work in ever more aggressive conditions, corrosion resistant coatings are often perceived to be the most viable solution.
Authors of other papers at this first ever Institute of Materials (10M) Mate- rials Congress will be examining the present status of materials in state-of-the-art power generation. Here we are concerned with a rather different set of materials, intended for higher temperatures or more aggressive conditions.
The difference between these advanced materials, and the current set of low alloy and stainless steels for power plant superheater and reheater applications, is best shown in the variety of strengthening mechanisms utilised. The full range of these will be explored in the text, but they include strengthening by oxide dis- persions, gamma prime, and complex carbides and nitrides. In terms of corrosion resistance, the advanced alloys are characterised by much higher levels of chromium, aluminium, silicon, and nickel than the common or garden stainless steels. Again, in this paper, we will be examining the principal environments that these advanced materials are intended to withstand, and indicating what factors govern good corrosion resistance.
In most types of energy conversion and process plant, the development of materials for steam boiler pipework and superheaters has been, and still is,
evolutionary rather than revolutionary. This is particularly true of electricity generation, where the increase in steam temperatures has been relatively slow, particularly over the last thirty years. To a certain extent, this has been caused by the need to ensure that other sections of the generating plant, for example steam turbine casings, rotors, and stop valves, etc, can take the higher steam condi- tions. The introduction of new materials will be faster if an increase of tem- peratures and pressures in a high temperature heat exchanger has no serious implications for the rest of the equipment. This is often the case in the oil refining and petrochemical sectors, where the upstream and downstream plant consists of pipework and low technology heat transfer equipment, rather than complex, highly stressed machinery.
This would suggest that there will be greater incentives to introduce a new material if the heat exchanger itself is of a new type, or if it is of a conventional form that is working under extreme conditions. In considering the prospects for advanced materials in heat exchangers, one needs to consider the prospects for advanced heat exchangers and processes themselves, and this will form the first section of this paper.
There is no set definition for what can be described as an advanced heat exchanger, but it is one that will be used in:
A simple development of a well known process, operating in conditions which are significantly more severe than in standard equipment.
A process which is extremely conventional in many respects, but requiring a fairly radical change in heat exchanger materials or design.
A completely new process, which is only viable as the direct result of im- provements in heat exchanger design, fabrication techniques, and/or new materials.
Obviously, these classifications overlap. For example, an advanced pulverised fuel steam plant, operating at 750°C and 450 bar pressure, would undoubtedly require a quantum leap in the performance of superheater materials. Some, however, would regard a pressurised fluidised bed boiler, as being a simple variation of conventional steam plant technology, despite the fact that it is subject to new forms of metals wastage. Even exchangers for indirect coal fired gas turbine cycles, which are only possible now that we have materials which can operate above 1100°C, in corrosive environments, would be seen by many of those who work in the petrochemical field, as being a straightforward develop- ment of steam reformer or ethylene plant technology.
Nevertheless, in the past decade, the desire for compact, high temperature heat exchangers has led to concepts which fall almost completely into the third classification. That is, heat exchanger designs and materials, without which a new system will fail. Of the concepts that are about to reach the market, the best example is that of the recuperative gas turbine. Whether in the form of the micro turbine giving out less than a 100 kW, or one on a rather larger scale for ship-
borne propulsion or industrial use, if this, and related developments are suc- cessful, we may be at the start of a new era in power generation and transport.
2 LESSONS OF HISTORY
Although most papers in this section of the Materials Congress focus on low alloy and bainitic steels for generating plant, most of the initiatives, for high temperature heat exchanger design, and for their materials of construction, have come from outside of the power generation sector. This history goes back a long way, almost to the start of the Industrial Revolution. The problems that en- gineers faced at that time were basically those we have today. The very first high temperature heat exchanger, the Neilson hot blast stove, was introduced just over 150 years ago, Fig. 1. This, by heating the charge air to a blast furnace to about 350°C, reduced coke consumption by a factor of 2, revolutionising iron making practice. 1
Neilson's stove used cast iron exchanger tubing, and two of the main problems which bedevilled the very first gas to gas heat exchanger, high temperature corrosion, and what we would regard as thermal stress, are still, in some respects, the major challenges in the advanced heat exchanger field.
Real progress in the high temperature heat exchanger field only recommenced, in the oil industry, during the 1920s, with the need for thermal cracking of high molecular weight hydrocarbons. Despite an absolute paucity of stress rupture data, low alloy carbon and austenitic stainless steels were used in pipe stills and other forms of oil heater, at metal temperatures of up to 800°C! Not surprisingly, sulphide corrosion and coke formation were critical issues. There are also in- dications that elevated temperature embrittlement, due to carburisation or sigma phase formation, was a problem' These are still major issues in the oil refining and petrochemical sectors.
The most dramatic step forward, in terms of really high temperature opera- tion, came about with the development of centrifugal casting of high carbon austenitic stainless steels in the late fortiea ' It is difficult to conceive of steam reforming or ethylene production without tubes of this type. Pressures in these
Fig. 1 Prototype Neilson Blast Stove, circa 1838. Blast air, at atmosphere pressure, is heated to about 350°C in an arch of cast iron tubes in furnace on right, and passed to blast furnace on left.1
~ 75,...---..
::c_o
.~ ~ 50 ,--_._-.---.---.."-.---~---'-~
E~
~ ~ 25
.a •...
a.-~
a:~ O~-..z-.c;..---~--~--_r__----I
~ 0 0.2 0.4 0.6 O.B
Carbon Content0/0
Fig. 2 Effect of carbon content on short term stress rupture life of 18Crj37Ni type spun cast stainless alloy. (Redrawn from Ref. 2)
processes are still low by power plant standards, even today not exceeding 40 bar for reforming and about 5 bar for ethylene plant, but metal temperatures are in the 900°-11 OO°Cregion. It was quickly determined that carbon levels of around 0.4-0.45% were optimum in terms of stress rupture properties, Fig. 2.
The main shortcoming of the early cast stainless steels was embrittlement due to the formation of fine carbides during service. The high chromium, low nickel, sigma forming alloys have had little use for pressurised tubing. Whilst pre- cipitation of carbides does not cause serious problems at working temperature, below 600°C ductility would fall to under 40/0, giving the risk of embrittlement and catastrophic failure. Initially, outlet headers on reforming plant were also made from cast stainless steels of 18Cr-37Ni type, but in the mid 1960s there were some very nasty accidents in the UK gas industry with this type of design.
At least two serious explosions were caused by severe thermal stressing of the outlet headers, during boiler priming incidents." Accordingly, cast headers quickly went out of fashion, being replaced by those of the wrought variety.
Having been involved in such a boiler priming 'incident', the writer can write with feeling about the efficacy of wrought headers.
The principal high temperature wrought alloy on offer, at that time, was Incoloy 800, itself a late 1940s material. This had been devised as an iron based substitute for the Inconel series. Chromium and nickel contents were set to give a good combination of high temperature strength and static high temperature oxidation resistance (that is in the absence of frequent temperature cycling), whilst avoiding the sigma phase propensity of conventional stainless steels such as Type 310. Incoloy was a key alloy in the history of high temperature materials, and was clearly formulated in the light of experience. Electron/atom ratio con- cepts, such as PHACOMP, to control the alloy composition, so as to prevent the formation of sigma, were not then in use. And obviously, at that time, computer prediction of phase diagrams was not even a pipe dream.
A major conference was held on the Incoloy alloys in Europe in 1978.5Incoloy 800 itself has long been superseded for high temperature use by Incoloy 800 HT,
also from the Inco stable, but this alloy now has many competitors, and some of these will be given separate consideration in this paper.
Incoloy 800 and most of the alloys of that time were, by modern standards, quite low in chromium. These materials were intended for use in high tempera- ture furnaces burning natural gas or naphtha, giving combustion products that were oxidising, low in sulphur dioxide and free from ash. Conversely, in a 1950s style conventional steam plant, where ash and S02 were present, there was little call for an alloy which could withstand fireside corrosion, since temperatures at that time were low. However, it is understood that a more resistant variety of Incoloy 800 was developed for an indirect fired closed cycle gas turbine."
It was in the UK a little later, in the 1960s, where the problem of fireside corrosion received most attention. UK coals are more aggressive than those on the Continent, and average steam temperatures, in this country, were at that time higher.7,s Some aspects of fireside corrosion will be covered in Section 5.3, since it will be a critical issue in the development of very advanced pulverised fuel plant.
Most forms of fossil fuel plant for power production can be expected to encounter thermal fatigue. This seems to have been a major reason why the generating industry has avoided cast stainless materials. At some time in the life of generating plant it will move to 'two shifting', so that it is essential that superheater tubing should retain high ductility and fracture toughness, even after long periods of service. Latterly there has been a real effort to develop cast stainless alloys with lower carbon contents, having enhanced resistance to em- brittlement. Despite this, there still seems to be a reluctance to consider the merits of the spun cast stainless tubing. None of these materials have been incorporated into very long term generating plant test programmes to date.
There is, it would seem, in the refinery and petrochemical fields, a greater willingness to introduce higher temperature materials, without long-term stress rupture and low cycle fatigue data to underpin their use. This is not to say that new high temperature alloys are used in a cavalier fashion, or that there are no institutional obstacles to overcome in their introduction. Manufacturers have to provide reasonably extensive stress rupture data for design purposes, but often this is extrapolated from medium term tests using log time-log stress, or simple parametric methods. A suitable safety factor is then applied to give a design stress. This makes for a conservative approach in the stressing of high tem- perature heaters and furnace tubing, and a number organisations, including ERA Technology, have become quite skilful at indicating how much extra life can be wrung out of apparently time expired equipment."
The power plant sector insists on ever more stringent criteria for new mate- rials, particularly when advanced processes are under consideration. When European organisations began to consider the prospects for nuclear heated gas turbine cycles and steam reforming processes, it was considered vital to put in hand major programmes on HK40, Incoloy 800, and Inconel 617, among other materials, even though some of these had been in commercial use for many years.
Today, even in the oil refining and processing sector, it seems more difficult to get new materials introduced than it was once. Health and Safety legislation is a major concern, and there is less tolerance of unscheduled shutdowns due to a new material not coming up to expectations. There is also, perhaps, a percep- tion that the newer alloys do not have much to offer compared to standard materials.
Despite these obstacles, mainstream alloy development is still continuing, and it is the arrival of processes, which require new types of heat exchanger, which give advanced materials their best opportunity. In exchanger designs which are close to the conventional, the possibility of introducing truly innovative tube alloyswill be low, particularly if there are misgivings of any kind about the newer materials. To help allay reasonable concerns, the paper will attempt to show how these new materials are superior to those of old. It is also intended, where possible, to indicate the thinking behind the new alloys, so that engineers and designers can have greater confidence in the claims of alloy developers.
3 ADVANCED HEAT EXCHANGER CONCEPTS FOR THE NINETIES AND BEYOND
As indicated in the Introduction, to give focus to the paper, a number of nearer term technologies that need heat exchange will be briefly described. These include indirect fired and recuperative gas turbines, very advanced pulverised fuel plant, fluidised bed combustion, coal gasification, and waste incineration. Table 1 summarises the technologies that require advanced heat exchange systems. The scale varies tremendously, from 100 kW up to 1000 MW.
3.1 Indirect Coal Fuelled Fired Gas Turbine
In an indirect fired gas turbine, the combustion chamber of the gas turbine is replaced by a large, externally fired, heat exchanger, into which flows pressurised air from a compressor. The air for the gas turbine is therefore heated 'indirectly' by the combustion of coal, biomass, or any other fuel, which is burnt on the furnace side of the exchanger. The pressurised air, now heated to a suitably high temperature, is expanded through the turbine section where it produces sufficient energy to drive a generator and the compressor.l'' Fig. 3.
In most types of indirect fired gas turbine cycle, the temperature to which the air must be heated has to be in excess of 1100°C, if these cycles are to be competitive with advanced pulverised combustion or fluidised bed coal plant.
Ideally, turbine inlet temperatures should be equal to that of the best natural gas fired combined cycle plant. This today is approaching 1450°C, but such a tem- perature would be impracticable with metallic materials of construction, and perhaps even with ceramics. Hence high efficiencies can only be obtained by using a relatively sophisticated thermodynamic cycle, and ensuring that com- ponent design is better than in an open cycle plant. Fortunately, these are quite