Materials giants Praxair (NYSE:PX) and Airgas (NYSE:ARG) have enjoyed great runs over the past year, with gains that meet or exceed the broader market. Meanwhile, Air Products and Chemicals (NYSE:APD) has produced even better returns, up more than 40% over the past 52 weeks.

With such great returns from traditionally slow-and-steady companies such as these, which are engaged in the relatively modest-growth business of producing industrial gases, investors may be understandably questioning whether these stocks have further room to run.

Two great companies, with not-so-great valuations
Praxair definitely fits the bill as a well-run, highly profitable company. At its 2013 Investor Day, Praxair proved as much with underlying fundamentals that place it at the forefront of its industry. During the 20 years between 1992 and 2012, Praxair has consistently produced above-industry average operating margins, including a 22% margin last year compared to just 15% for the industry. This high level of operating efficiency was a primary driver behind the company's ability to produce 12% growth in operating cash flow compounded annually over the past two decades.

Going forward, Praxair fully expects its strong results to continue. Its five-year outlook is for 8% compound annual growth in sales. This will be achieved through a combination of favorable pricing, new projects, as well as acquisitions. All told, its goal is to reach $16 billion in sales by 2017, while achieving at least 14% return on capital every year.

It's a similar story for Airgas, which is reaping the benefits of a highly diversified business. Airgas delivers its specialty gases to a wide range of industries. In total, Airgas has more than one million customers, and no single customer accounts for more than a half of one percent of the company's total revenue. This has allowed Airgas to produce stable operating results, even though it's a cyclical company highly tied to the swings of the broader economy.

That latter statement is actually a reason for optimism for Airgas. The company believes the U.S. economic recovery, although slow, is gaining traction. That will mean good things for the company's main operating segment, strategic products, which accounts for 40% of the company's revenue. This segment produces safety products, bulk gases, and medical sales, and should produce strong results if the economy is indeed on the upswing.

Is this industry giant falling behind?
At the same time, Air Products surely generated a lot of hype in the wake of a sizable investment from noted money manager Bill Ackman, but its underlying business leaves a lot to be desired. The company's 2012 sales dropped 1% versus the prior year, and the company barely eked out 1% growth in diluted earnings per share. Unfortunately, metrics across the board deteriorated for Air Products last year, including compressed return on capital, return on equity, and operating margin.

And, 2013 is projected to be another year of little to no growth. Air Products expects to generate at least $5.47 in diluted earnings per share, which would represent growth of just a half of one percent year over year.

Are these stocks full of hot air?
As solid as Airgas and Praxair are, their stocks appear to now fully reflect this (and then some). And, in Air Products' case, there are critical questions about future growth that need to be answered. At recent prices, each of these stocks trades for nearly 25 times trailing earnings, lofty valuations that are well ahead of the S&P 500 Index as a whole.

Furthermore, each of these three stocks is trading at multiples near their highs in the last five years. While you can't argue with the returns generated over the past year, at this point it seems these stocks may have gotten a little ahead of themselves. Future growth is expected to be stable but modest, meaning it may be wise for investors to wait for a pullback before jumping in to Airgas, Praxair, and Air Products.

Bob Ciura has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.