Despite the divestitures, orders were up over 10%.
On an organic basis, our sales were up 5.5%.
Our first-quarter adjusted EBITDA of $52 million increased 28% year over year, and adjusted EBITDA margin expanded 420 basis points to 18.6%.
The Advanced Surface Technologies segment, which includes Alluxa, LeanTeq and the Technetics Semiconductor businesses, posted 49% revenue growth, 137% adjusted EBITDA growth, and adjusted EBITDA margin expansion of over 1,000 basis points to 31.6%.
We anticipate continued strong demand for the remainder of the year.
The percentage of female promotions in the U.S. has increased 10% since January 2019.
In the area of supporting our communities, we announced in December our $1 million funding of the EnPro Foundation focused on advancing education, equality, diversity and the preservation of human dignity.
Currently, approximately 7% of our revenue comes from the oil and gas industry, and we anticipate this percentage to decrease over time as our strategic transformation continues.
As reported, sales of $279 million for the first quarter decreased 1.2% year over year.
Excluding the impact of foreign exchange translation and sales from acquired and divested businesses, organic sales for the quarter grew 5.5% compared to the first quarter of 2020.
Sequentially, excluding portfolio reshaping activities, sales were up 7.1%.
Gross profit margin of 39.2% increased 550 basis points versus the prior-year period.
The year-over-year improvement in gross profit margin was achieved despite a $2.4 million amortization of acquisition-related inventory write-up in the first quarter of 2021.
Adjusted EBITDA of $52 million increased 28.1% over the prior-year period as a result of organic sales growth, strategic acquisitions and cost reductions taken across the company.
Adjusted EBITDA margin of 18.6% increased approximately 420 basis points compared to the first quarter of 2020.
Corporate expenses of $11.6 million in the first quarter of 2021 increased by $3.2 million from last year.
Adjusted diluted earnings per share of $1.37 increased 43% compared to the prior-year period.
Amortization of acquisition-related intangible assets in the first quarter was $11.3 million compared to $9 million in the prior-year period.
We anticipate amortization of acquisition-related intangibles will be between $44 million and $46 million in 2021.
As a reminder, our estimated normalized tax rate used in determining adjusted earnings per share is 30%.
Moving to the discussion of segment performance, Sealing Technologies, which includes Garlock, STEMCO and the Technetics sealing businesses, reported sales of $146.5 million in the first quarter.
The year-over-year decrease of 15.6% was due to portfolio reshaping activities last year.
Excluding the impact of foreign exchange translation and sales from divested businesses, sales increased 0.9% versus the prior-year period.
For the first quarter, adjusted EBITDA increased 1.2% to $33.9 million and adjusted segment EBITDA margin expanded 380 basis points to 23.1%.
Excluding the impact of foreign exchange translation and divestitures, adjusted segment EBITDA increased 6.1% compared to the prior-year period.
Turning now to Advanced Surface Technologies, which includes Alluxa, LeanTeq and the Technetics Semiconductor businesses, first quarter sales of $54.7 million increased to 49%, driven primarily by strong demand in the semiconductor market and the acquisition of Alluxa.
Excluding the impact of foreign exchange translation and the Alluxa acquisition, sales increased 22.6% versus the prior-year period.
For the first quarter, adjusted segment EBITDA increased 137% to $17.3 million, and adjusted segment EBITDA margin expanded from 19.9% a year ago to 31.6%, driven primarily by the Alluxa contribution and growth in the LeanTeq business.
Excluding the impact of Alluxa and foreign exchange translation, adjusted segment EBITDA increased 57.5% compared to the prior-year period.
In Engineered Materials, which consists of GGB and CPI, first quarter sales of $80.4 million increased 7.1% compared to the prior year, driven by stronger sales in general industrial, automotive and petrochemical markets, partially offset by weakness in the oil and gas and aerospace markets.
Excluding the impact of foreign exchange translation and the divestiture of GGB's Bushing Block business, sales for the quarter increased 5.3%.
For the first quarter compared to previous year, adjusted segment EBITDA increased 51.8% to $12.6 million, and adjusted segment EBITDA margin expanded 460 basis points to 15.7%.
Excluding the impact of foreign exchange translation, adjusted EBITDA increased 39.3% compared to the prior-year period.
We ended the quarter with cash of $232 million and with full availability on our $400 million revolver, less $11 million in outstanding letters of credit.
At the end of March, our net debt to adjusted EBITDA ratio was approximately 1.4 times, a sequential decline from the 1.6 times reported at the end of the fourth quarter.
Free cash flow for the quarter was $14.1 million, up from negative $4.9 million in the prior year.
During the first quarter, we paid a $0.27 per share quarterly dividend totaling $5.7 million, a 4% increase versus the prior year.
Moving now to 2021 guidance, taking into consideration all the factors that we know at this moment, including the ongoing global economic recovery from the COVID-19 pandemic which is stronger-than-anticipated a quarter ago, we are increasing 2021 adjusted EBITDA to be in the range of 190 million to $200 million, up from our previous guidance of $178 million to $188 million.
The updated adjusted EBITDA range is based on sales growth of 7 to 12% over 2020 pro forma sales of $983 million, up from previous guidance range of 6 to 10% growth.
We expect adjusted diluted earnings per share from continuing operations to be in the range of $4.74 to $5.08, up from the range of $4.32 to $4.66 provided last quarter.
Our guidance assumes depreciation and amortization expense, excluding amortization of acquisition-related intangible assets, in the range of $33 million and $35 million and net interest expense of $15 million to $17 million, both unchanged from prior guidance.

Summary:
We anticipate continued strong demand for the remainder of the year.
Adjusted diluted earnings per share of $1.37 increased 43% compared to the prior-year period.
We expect adjusted diluted earnings per share from continuing operations to be in the range of $4.74 to $5.08, up from the range of $4.32 to $4.66 provided last quarter.